Add 'is_chrome_branded' guard to the default of 'rtc_use_h264'

This doesn't change behavior at the moment because Chromium's
'proprietary_codecs' is already conditional on 'is_chrome_branded'
but this guards WebRTC's default from upstream changes like
https://chromium-review.googlesource.com/c/chromium/src/+/835010/6/build/config/features.gni

TBR=phoglund@webrtc.org

Bug: webrtc:8675
Change-Id: Ic2ae311b5fc70a4d1ac1aefe4cc27574e4fcee40
Reviewed-on: https://webrtc-review.googlesource.com/36321
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21452}
diff --git a/webrtc.gni b/webrtc.gni
index fc9eadb..986598b 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -149,7 +149,8 @@
   # also: |rtc_initialize_ffmpeg|.
   # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
   # http://www.openh264.org, https://www.ffmpeg.org/
-  rtc_use_h264 = proprietary_codecs && !is_android && !is_ios
+  rtc_use_h264 =
+      is_chrome_branded && proprietary_codecs && !is_android && !is_ios
 
   # By default, use normal platform audio support or dummy audio, but don't
   # use file-based audio playout and record.