Implement MediaSource VideoPlaybackQuality functionality.

Implements the VideoPlaybackQuality object and HTMLVideoElement.getVideoPlaybackQuality()
method specified in the Media Source Extensions spec. This change implements all attributes
mentioned in the spec except for totalFrameDelay because that attribute is currently
being debated. Only stable attributes that aren't expected to change are being implemented here.

BUG=313406
TEST=LayoutTests/http/test/media/media-source/mediasource-getvideoplaybackquality.html

Review URL: https://codereview.chromium.org/52793003

git-svn-id: svn://svn.chromium.org/blink/trunk@161100 bbb929c8-8fbe-4397-9dbb-9b2b20218538