Fix for fallback codepath for WebGL to 2D canvas pixel copying

BUG=329949
R=aelias@chromium.org, senorblanco@chromium.org

Review URL: https://codereview.chromium.org/137323003

git-svn-id: svn://svn.chromium.org/blink/trunk@165475 bbb929c8-8fbe-4397-9dbb-9b2b20218538