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/webrtc/api/audio_codecs/opus/BUILD.gn b/webrtc/api/audio_codecs/opus/BUILD.gn
index 29a68ff..c7f7ac8 100644
--- a/webrtc/api/audio_codecs/opus/BUILD.gn
+++ b/webrtc/api/audio_codecs/opus/BUILD.gn
@@ -18,7 +18,7 @@
     "audio_encoder_opus_config.h",
   ]
   deps = [
-    "../../../rtc_base:rtc_base_approved",
+    "../../../base:rtc_base_approved",
   ]
   defines = []
   if (rtc_opus_variable_complexity) {
@@ -35,9 +35,9 @@
   deps = [
     ":audio_encoder_opus_config",
     "..:audio_codecs_api",
+    "../../../base:protobuf_utils",  # TODO(kwiberg): Why is this needed?
+    "../../../base:rtc_base_approved",
     "../../../modules/audio_coding:webrtc_opus",
-    "../../../rtc_base:protobuf_utils",  # TODO(kwiberg): Why is this needed?
-    "../../../rtc_base:rtc_base_approved",
   ]
 }
 
@@ -49,7 +49,7 @@
   deps = [
     "..:audio_codecs_api",
     "../../..:webrtc_common",
+    "../../../base:rtc_base_approved",
     "../../../modules/audio_coding:webrtc_opus",
-    "../../../rtc_base:rtc_base_approved",
   ]
 }