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/rtc_base/thread.h b/rtc_base/thread.h
index 6e68f1a..cb5a0e9 100644
--- a/rtc_base/thread.h
+++ b/rtc_base/thread.h
@@ -400,15 +400,17 @@
 
   // Allows invoke to specified |thread|. Thread never will be dereferenced and
   // will be used only for reference-based comparison, so instance can be safely
-  // deleted. If NDEBUG is defined and DCHECK_ALWAYS_ON is undefined do nothing.
+  // deleted. If NDEBUG is defined and RTC_DCHECK_ALWAYS_ON is undefined do
+  // nothing.
   void AllowInvokesToThread(Thread* thread);
 
-  // If NDEBUG is defined and DCHECK_ALWAYS_ON is undefined do nothing.
+  // If NDEBUG is defined and RTC_DCHECK_ALWAYS_ON is undefined do nothing.
   void DisallowAllInvokes();
   // Returns true if |target| was allowed by AllowInvokesToThread() or if no
   // calls were made to AllowInvokesToThread and DisallowAllInvokes. Otherwise
   // returns false.
-  // If NDEBUG is defined and DCHECK_ALWAYS_ON is undefined always returns true.
+  // If NDEBUG is defined and RTC_DCHECK_ALWAYS_ON is undefined always returns
+  // true.
   bool IsInvokeToThreadAllowed(rtc::Thread* target);
 
   // Posts a task to invoke the functor on |this| thread asynchronously, i.e.