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TimeSource represents the canonical source of time and will eventually
replace the time-updating callback from AudioRenderer and VideoRenderer.

A wall-clock based implementation is also provided for media pipelines
that do not contain any audio.

BUG=370634

Review URL: https://codereview.chromium.org/379343005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284122 0039d316-1c4b-4281-b951-d872f2087c98