Fix copyImage for WebGL elements.

"Copy image" menu item crashes on WebGL elements because the
copiedImage in HTMLCanvasElement gets cleared before it can
be copied to the clipboard.
Also fixed HTMLCanvasElement::copiedImage() so that the copy
is done on the latest frame composited to the screen.

BUG=392765

Review URL: https://codereview.chromium.org/389273002

git-svn-id: svn://svn.chromium.org/blink/trunk@178508 bbb929c8-8fbe-4397-9dbb-9b2b20218538