Test Canvas tainting and WebGL video textures (CORS fetched.)

Verify that cross-origin video resources follow the required Canvas
tainting rules when included in textures via texImage2D().

R=kbr
BUG=173727

Review URL: https://codereview.chromium.org/80263004

git-svn-id: svn://svn.chromium.org/blink/trunk@162487 bbb929c8-8fbe-4397-9dbb-9b2b20218538