libwebp: cherry-pick some demux changes

these make the demuxer more strict when parsing files with VP8X flags that
don't match the data.

e18e667 demux: strictly enforce the animation flag
61cb884 demux: (non-exp) fail if the fragmented flag is set

BUG=317284
R=urvang@google.com
TBR=fbarchard

Review URL: https://codereview.chromium.org/60863004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235867 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235867