Default build flag |rtc_use_h264| to |proprietary_codecs| if not on Android/iOS.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index 886b8995433635f8c6f21c353197114e17bef154..90ec934164b14315dde4c648aa3435d5141acedf 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -7,6 +7,7 @@
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
+import("//build/config/features.gni")
 import("//build/config/mips.gni")
 import("//build_overrides/webrtc.gni")
 
@@ -92,14 +93,14 @@ declare_args() {
   rtc_use_objc_h264 = false
 
   # Enable this to build OpenH264 encoder/FFmpeg decoder. This is supported on
-  # all platforms except iOS. Because FFmpeg can be built with/without H.264
-  # support, |ffmpeg_branding| has to separately be set to a value that
-  # includes H.264, for example "Chrome". If FFmpeg is built without H.264,
-  # compilation succeeds but |H264DecoderImpl| fails to initialize.
-  # See also: |rtc_initialize_ffmpeg|.
+  # all platforms except Android and iOS. Because FFmpeg can be built
+  # with/without H.264 support, |ffmpeg_branding| has to separately be set to a
+  # value that includes H.264, for example "Chrome". If FFmpeg is built without
+  # H.264, compilation succeeds but |H264DecoderImpl| fails to initialize. See
+  # 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 = false
+  rtc_use_h264 = proprietary_codecs && !is_android && !is_ios
 
   # FFmpeg must be initialized for |H264DecoderImpl| to work. This can be done
   # by WebRTC during |H264DecoderImpl::InitDecode| or externally. FFmpeg must