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/webrtc/sdk/objc/Framework/UnitTests/RTCTracingTest.mm b/webrtc/sdk/objc/Framework/UnitTests/RTCTracingTest.mm
index ec3e226..49cc812 100644
--- a/webrtc/sdk/objc/Framework/UnitTests/RTCTracingTest.mm
+++ b/webrtc/sdk/objc/Framework/UnitTests/RTCTracingTest.mm
@@ -12,7 +12,7 @@
 
 #include <vector>
 
-#include "webrtc/base/gunit.h"
+#include "webrtc/rtc_base/gunit.h"
 
 #import "NSString+StdString.h"
 #import "WebRTC/RTCTracing.h"