Support format using enum argument for Async readback.

As of now we are passing bool to detect the format like rgb565, but ideally the format itself should be passed.

BUG=323150

Review URL: https://codereview.chromium.org/143683003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246888 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246888