Remove GraphicsContextCG.

We haven't supported Core Graphics in some time.
Only one of these functions is implemented at all, and it  is unused.

BUG=381789

Review URL: https://codereview.chromium.org/315403007

git-svn-id: svn://svn.chromium.org/blink/trunk@175724 bbb929c8-8fbe-4397-9dbb-9b2b20218538