skip (gpu) canvas-toDataURL-webp until libwebp-0.4.1 lands

BUG=374343

Review URL: https://codereview.chromium.org/413403010

git-svn-id: svn://svn.chromium.org/blink/trunk@179090 bbb929c8-8fbe-4397-9dbb-9b2b20218538