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/2976293002
Cr-Commit-Position: refs/heads/master@{#19094}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index ca6a70a..d29c4c4 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -18,7 +18,6 @@
   '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',
@@ -74,7 +73,6 @@
 # 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',
@@ -91,6 +89,7 @@
   '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',
 )