WebP: Regression test for incremental decoding

Add a test for http://crbug.com/364830

BUG=364830

Review URL: https://codereview.chromium.org/255523004

git-svn-id: svn://svn.chromium.org/blink/trunk@172569 bbb929c8-8fbe-4397-9dbb-9b2b20218538