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

webrtc/build/webrtc.gni

 # Copyright (c) 2014 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.
 
 import("//build/config/arm.gni")
+import("//build/config/features.gni")
 import("//build/config/mips.gni")
 import("//build_overrides/webrtc.gni")
 
 declare_args() {
   build_with_libjingle = true
 
   # Disable this to avoid building the Opus audio codec.
   rtc_include_opus = true
 
   # Used to specify an external Jsoncpp include path when not compiling the
@@ skipping 65 matching lines @@
   # Determines whether NEON code will be built.
   rtc_build_with_neon =
       (current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon)) ||
       current_cpu == "arm64"
 
   # Enable this to use HW H.264 encoder/decoder on iOS PeerConnections.
   # Enabling this may break interop with Android clients that support H264.
   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
   # only be initialized once. Projects that initialize FFmpeg externally, such
   # as Chromium, must turn this flag off so that WebRTC does not also
   # initialize.
   rtc_initialize_ffmpeg = !build_with_chromium
 }
 
 # A second declare_args block, so that declarations within it can
 # depend on the possibly overridden variables in the first
 # declare_args block.
 declare_args() {
   # Include the iLBC audio codec?
   rtc_include_ilbc = !(build_with_chromium || build_with_mozilla)
 }
 
 # Make it possible to provide custom locations for some libraries (move these
 # up into declare_args should we need to actually use them for the GN build).
 rtc_libvpx_dir = "//third_party/libvpx_new"
 rtc_libyuv_dir = "//third_party/libyuv"
 rtc_opus_dir = "//third_party/opus"