WebPDecoder: Detect truncated file at the time of parsing

We should fail when we have received all data, but WebPDemuxPartial()
hasn't reached the state WEBP_DEMUX_DONE.

Added a unit test for the same.

This is a fix for: https://code.google.com/p/webp/issues/detail?id=185

BUG=

Review URL: https://codereview.chromium.org/144553003

git-svn-id: svn://svn.chromium.org/blink/trunk@165678 bbb929c8-8fbe-4397-9dbb-9b2b20218538