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First step in transitioning HardwareDisplayController to take in buffers rather
than surfaces. ScanoutBuffer defines the minimal interface required by HDC for
modesetting and page-flipping.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/399213005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284115 0039d316-1c4b-4281-b951-d872f2087c98