Prefer to forward-declare GraphicsContext in headers.

Then clean up the errors that result from changing that.

For me, this reduces the number of targets built on a trivial change to
GraphicsContext.h from 888 to 297.

Review URL: https://codereview.chromium.org/319183004

git-svn-id: svn://svn.chromium.org/blink/trunk@175919 bbb929c8-8fbe-4397-9dbb-9b2b20218538