Fix imageSourceURL() for WebGL elements.

For saveImageAs, we want to save the latest frame that has 
been composited to the screen. So in this case, copy from the
frontBuffer in DrawingBuffer::copyToPlatformTexture.
    
BUG=374762

Review URL: https://codereview.chromium.org/309933004

git-svn-id: svn://svn.chromium.org/blink/trunk@176007 bbb929c8-8fbe-4397-9dbb-9b2b20218538