libwebp: cherry-pick upstream format validation in demux

dde91fd Demux: Correct the extended format validation

Unit test for this being added in http://crrev.com/35893011

BUG=310257
R=fbarchard@chromium.org

Review URL: https://codereview.chromium.org/39183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230666 0039d316-1c4b-4281-b951-d872f2087c98
Automated Commit: Committing new LKGM version 4861.0.0 for chromeos.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230667 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230667