MediaStreamRecorder: enable in Android (including linking libwebm)

This follows a conversation in the Intent-To-Ship [1].

The added code means:
- third_party/libwebm
- media::WebmMuxer
- content::AudioTrackRecorder
- content::VideoTrackRecorder
- content::MediaRecorderHandler.

third_party/libwebm size: 10154B

Increase in ChromePublic-unsigned.apk:
54673026 - 54651438 = 21588B ~= 21KB

----- Note:
Modified code in webm_muxer.cc due to complaint from
arm-linux-androideabi-4.9:
../../media/capture/webm_muxer.cc:49:76: error: array subscript is above array bounds [-Werror=array-bounds]
            kOpusVorbisChannelMap[params.channels() - 1], params.channels());
------

BUG=262211, 561068

TEST=Same as for desktop platform:
https://rawgit.com/Miguelao/demos/master/mediarecorder.html
Results of running it can be found under [2] and [3]

[1] https://groups.google.com/a/chromium.org/d/msg/blink-dev/76HB0BIxk_o/LtZYZWvVBQAJ
[2] https://drive.google.com/file/d/0B82Jhdx0kSTVS3E4bFZwZUxSSTQ/view?usp=sharing
[3] https://drive.google.com/file/d/0B82Jhdx0kSTVM3lfR2N0RFlwNzg/view?usp=sharing

Review URL: https://codereview.chromium.org/1486623004

Cr-Commit-Position: refs/heads/master@{#362536}

1 files changed, 4 insertions, 2 deletions

diff --git a/media/capture/webm_muxer.cc b/media/capture/webm_muxer.cc
index 5e569bc..939cb39 100644
--- a/media/capture/webm_muxer.cc
+++ b/media/capture/webm_muxer.cc
@@ -45,8 +45,10 @@ void WriteOpusHeader(const media::AudioParameters& params, uint8* header) {
     header[OPUS_EXTRADATA_NUM_STREAMS_OFFSET] = params.channels();
     header[OPUS_EXTRADATA_NUM_COUPLED_OFFSET] = 0;
     // Set the actual stream map.
-    memcpy(header + OPUS_EXTRADATA_STREAM_MAP_OFFSET,
-           kOpusVorbisChannelMap[params.channels() - 1], params.channels());
+    for (int i = 0; i < params.channels(); ++i) {
+      header[OPUS_EXTRADATA_STREAM_MAP_OFFSET + i] =
+          kOpusVorbisChannelMap[params.channels() - 1][i];
+    }
   } else {
     header[OPUS_EXTRADATA_CHANNEL_MAPPING_OFFSET] = 0;
   }