media/capture: Adding WebmMuxer class and unittests

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index cd7440a8db696cd32aaa9f652cd1646ac9063f06..a3b3a8a6a6834c269e6f301973cccb93e9433ca6 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -17,10 +17,14 @@ declare_args() {
   # decoding of VP9 and VP8A type content.
   media_use_libvpx = true
 
-  # Neither Android nor iOS use ffmpeg or libvpx.
+  # Enable libwebm for multiplexing video and audio for JS recording API.
+  media_use_libwebm = true
+
+  # Neither Android nor iOS use ffmpeg, libvpx nor libwebm.
   if (is_android || is_ios) {
     media_use_ffmpeg = false
     media_use_libvpx = false
+    media_use_libwebm = false
   }
 
   # Override to dynamically link the cras (ChromeOS audio) library.