Revert of Remove remains of webrtc/base (patchset #7 id:120001 of https://codereview.webrtc.org/2973183002/ )

Reason for revert:
Breaks lots of downstream projects.

Original issue's description:
> Remove remains of webrtc/base
>
> All downstream code have been updated to the new location.
>
> In PRESUBMIT.py:
> * Remove webrtc/rtc_base from CPP_BLACKLIST
> * Add webrtc/rtc_base to LEGACY_API_DIRS
>
> Fix some duplicated paths in
> webrtc/modules/audio_processing/test/conversational_speech/BUILD.gn
>
> BUG=webrtc:7634
> TBR=kwiberg@webrtc.org
>
> Review-Url: https://codereview.webrtc.org/2973183002
> Cr-Commit-Position: refs/heads/master@{#18948}
> Committed:
https://chromium.googlesource.com/external/webrtc/+/9483b49bafc681a8360dff7217e7651a74dea71d

TBR=kwiberg@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7634

Review-Url: https://codereview.webrtc.org/2976633002
Cr-Commit-Position: refs/heads/master@{#18949}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 2a22d73..3cc90cc 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -18,6 +18,7 @@
   'tools_webrtc',
   'webrtc/api/video_codecs/video_decoder.h',
   'webrtc/api/video_codecs/video_encoder.h',
+  'webrtc/base',
   'webrtc/examples/objc',
   'webrtc/media',
   'webrtc/modules/audio_coding',
@@ -73,6 +74,7 @@
 # These directories should not be used but are maintained only to avoid breaking
 # some legacy downstream code.
 LEGACY_API_DIRS = (
+  'webrtc/base',
   'webrtc/common_audio/include',
   'webrtc/modules/audio_coding/include',
   'webrtc/modules/audio_conference_mixer/include',
@@ -89,7 +91,6 @@
   'webrtc/modules/video_coding/codecs/vp8/include',
   'webrtc/modules/video_coding/codecs/vp9/include',
   'webrtc/modules/video_coding/include',
-  'webrtc/rtc_base',
   'webrtc/system_wrappers/include',
   'webrtc/voice_engine/include',
 )