Add UMA stats for RTC encoder. Also rename the init stat for RTC decoder.

Renaming UMA for RTC decoder from "Status" to "Success" to prevent
confusion with Media.GpuVideoDecoderInitializeStatus. The latter is an enum
and indicates success via "0" bucket, while
Media.RTCVideoDecoderInitDecodeStatus indicates success via "1".
This should hopefully help prevent confusion.

BUG=297492
TEST=manual run

Review URL: https://codereview.chromium.org/28013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229973 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229973