media/capture: Adding WebmMuxer class and unittests

media/media_variables.gypi

Index: media/media_variables.gypi
diff --git a/media/media_variables.gypi b/media/media_variables.gypi
index f359e36ad8cde2d5e90a09d3d8a31eaf61af774b..b203ca09d67e6f2f2d403919a5f1a2d94364b699 100644
--- a/media/media_variables.gypi
+++ b/media/media_variables.gypi
@@ -7,14 +7,16 @@
   '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,
       }],
     ],
   },