media/capture: Adding WebmMuxer class and unittests

media/media_variables.gypi

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Common media variables.
 {
   'variables': {
     'conditions': [
       ['OS == "android" or OS == "ios"', {
-        # Android and iOS don't use FFmpeg or libvpx by default.
+        # Android and iOS don't use FFmpeg, libvpx nor libwebm by default.
         # Set media_use_ffmpeg=1 for Android builds to compile experimental
         # support for FFmpeg and the desktop media pipeline.
         'media_use_ffmpeg%': 0,
         'media_use_libvpx%': 0,
+        'media_use_libwebm%': 0,
       }, {
         'media_use_ffmpeg%': 1,
         'media_use_libvpx%': 1,
+        'media_use_libwebm%': 1,
       }],
     ],
   },
 }