Set textures to cleared in VideoDecodeAccelerator::PictureReady.

Instead of clearing the textures in GVDA::AssignPictureBuffers, set
them to cleared in PictureReady. This can make OnAssignPictureBuffers
faster. This can reduce the latency when playing a video or starting
webrtc. This also make resolution change faster.

The test was done on Daisy running apprtc.appspot.com with 720p
decode. AssignPictureBuffers is reduced from 142ms to 24ms.
The decode time of the first three frames are reduced from
202ms, 163ms, 144ms to 74ms, 36ms, 26ms respectively.

BUG=298176
TEST=Play video. Run apprtc loopback. Run vda unittest.
Run resolution change in DASH youtube. Test raw resolution switch.

Review URL: https://codereview.chromium.org/24762003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226492 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226492