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This is the first patch in a sequence of changes to enable Pipeline
to properly manage underflows. This change allows the VideoRendererImpl
to be prerolled in non-seek situations.

BUG=144683
TEST=VideoRendererImplTest.Rebuffer, VideoRendererImplTest.Rebuffer_AlreadyHaveEnoughFrames

Review URL: https://codereview.chromium.org/89673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238513 0039d316-1c4b-4281-b951-d872f2087c98