Remove unused enum declaration ContextWebkit3d from HTMLCanvasElement::getContext()

webkit-3d is no longer used in getContext and hence should be removed.

Review URL: https://codereview.chromium.org/488433003

git-svn-id: svn://svn.chromium.org/blink/trunk@180853 bbb929c8-8fbe-4397-9dbb-9b2b20218538