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

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 3ce1b2fc570fcffbc080db743ef8ca12182821d8..738333d660fcbc0d6f1ec37e3dae9a22e120794e 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -138,17 +138,21 @@
     # Enabling this may break interop with Android clients that support H264.
     'use_objc_h264%': 0,
 
-    # 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|.
-    # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
-    # http://www.openh264.org, https://www.ffmpeg.org/
-    'rtc_use_h264%': 0,
-
     'conditions': [
+      # Enable this to build OpenH264 encoder/FFmpeg decoder. This is supported
+      # on 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/
+      ['proprietary_codecs==1 and OS!="android" and OS!="ios"', {
+        'rtc_use_h264%': 1,
+      }, {
+        'rtc_use_h264%': 0,
+      }],
+
       # FFmpeg must be initialized for |H264DecoderImpl| to work. This can be
       # done by WebRTC during |H264DecoderImpl::InitDecode| or externally.
       # FFmpeg must only be initialized once. Projects that initialize FFmpeg