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

webrtc/build/common.gypi

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # This file contains common settings for building WebRTC components.
 
@@ skipping 120 matching lines @@
     # enable schannel on windows.
     'use_legacy_ssl_defaults%': 0,
 
     # Determines whether NEON code will be built.
     'build_with_neon%': 0,
 
     # Enable this to use HW H.264 encoder/decoder on iOS/Mac PeerConnections.
     # 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,
+      }],
 
-    'conditions': [
       # 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
       # externally, such as Chromium, must turn this flag off so that WebRTC
       # does not also initialize.
       ['build_with_chromium==0', {
         'rtc_initialize_ffmpeg%': 1,
       }, {
         'rtc_initialize_ffmpeg%': 0,
       }],
@@ skipping 317 matching lines @@
           # of a more specific macro.
           'defines': [
             'WEBRTC_POSIX',
           ],
         }],
       ],
     },
   }, # target_defaults
 }