Reland "Add WebRTC specific dcheck_always_on."

This is a reland of 9f2a20f4342a3e86e1f9fdfe6f3d76fb539d41c2

See https://webrtc-review.googlesource.com/c/src/+/226563/1..2
for the fix. RTC_DCHECK_ALWAYS_ON needs to be in public_configs
in order to be propagated together with header #includes and
avoid ODR violations.

Original change's description:
> Add WebRTC specific dcheck_always_on.
>
> Inspired by V8 CL: crrev.com/c/3038528.
>
> This makes the WebRTC's dcheck control independent of Chromium's and
> prepares switching Chromium's default behavior without affecting
> WebRTC developers or builders.
>
> Preparation for: https://crrev.com/c/2893204
>
> Bug: chromium:1225701, webrtc:12988
> Change-Id: Ia0d21f9fb8e9d7704fd1beca16504c301a263b3b
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226465
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Markus Handell <handellm@webrtc.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#34512}

Bug: chromium:1225701, webrtc:12988
Change-Id: I1f78587487ee7b1a4a07b8c91b737a9e797b2323
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226563
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34519}
diff --git a/tools_webrtc/coverage/generate_ios_coverage_command.py b/tools_webrtc/coverage/generate_ios_coverage_command.py
index e01a75c..ff17c58 100644
--- a/tools_webrtc/coverage/generate_ios_coverage_command.py
+++ b/tools_webrtc/coverage/generate_ios_coverage_command.py
@@ -89,7 +89,7 @@
     target_cpu = 'x64' if is_simulator else 'arm64'
     return ([] + ['target_os="ios"'] + ['target_cpu="%s"' % target_cpu] +
             ['use_clang_coverage=true'] + ['is_component_build=false'] +
-            ['dcheck_always_on=true'])
+            ['rtc_dcheck_always_on=true'])
 
 
 def GenerateIOSSimulatorCommand():
@@ -103,7 +103,7 @@
                     [FormatIossimTest(t, is_xctest=True) for t in XC_TESTS] +
                     [FormatIossimTest(t, is_xctest=False) for t in TESTS])
 
-    print 'To get code coverage using iOS simulator just run following commands:'
+    print 'To get code coverage using iOS sim just run following commands:'
     print ''
     print ' '.join(gn_cmd)
     print ''
diff --git a/tools_webrtc/mb/docs/user_guide.md b/tools_webrtc/mb/docs/user_guide.md
index 8c66cd3..a8857d2 100644
--- a/tools_webrtc/mb/docs/user_guide.md
+++ b/tools_webrtc/mb/docs/user_guide.md
@@ -248,8 +248,8 @@
   }
   'mixins': {
     'bot': {
-      'gyp_defines': 'use_goma=1 dcheck_always_on=0',
-      'gn_args': 'use_goma=true dcheck_always_on=false',
+      'gyp_defines': 'use_goma=1 rtc_dcheck_always_on=0',
+      'gn_args': 'use_goma=true rtc_dcheck_always_on=false',
     },
     'debug': {
       'gn_args': 'is_debug=true',
@@ -267,8 +267,8 @@
       'gyp_defines': 'component=shared_library',
     },
     'trybot': {
-      'gyp_defines': 'dcheck_always_on=1',
-      'gn_args': 'dcheck_always_on=true',
+      'gyp_defines': 'rtc_dcheck_always_on=1',
+      'gn_args': 'rtc_dcheck_always_on=true',
     }
   }
 }
@@ -276,7 +276,7 @@
 
 and you ran `mb gen -c linux_release_trybot //out/Release`, it would
 translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
-`"use_goma=true dcheck_always_on=false dcheck_always_on=true"`.
+`"use_goma=true rtc_dcheck_always_on=false rtc_dcheck_always_on=true"`.
 
 (From that you can see that mb is intentionally dumb and does not
 attempt to de-dup the flags, it lets gyp do that).
diff --git a/tools_webrtc/mb/mb_config.pyl b/tools_webrtc/mb/mb_config.pyl
index 253a57a..74678f8 100644
--- a/tools_webrtc/mb/mb_config.pyl
+++ b/tools_webrtc/mb/mb_config.pyl
@@ -459,8 +459,8 @@
       'gn_args': 'is_clang=true',
     },
 
-    'dcheck_always_on': {
-      'gn_args': 'dcheck_always_on=true',
+    'rtc_dcheck_always_on': {
+      'gn_args': 'rtc_dcheck_always_on=true',
     },
 
     'debug': {
@@ -548,11 +548,11 @@
     },
 
     'release_bot': {
-      'mixins': ['pure_release_bot', 'dcheck_always_on'],
+      'mixins': ['pure_release_bot', 'rtc_dcheck_always_on'],
     },
 
     'release_bot_no_goma': {
-      'mixins': ['release', 'no_goma', 'dcheck_always_on'],
+      'mixins': ['release', 'no_goma', 'rtc_dcheck_always_on'],
     },
 
     'tsan': {