Remove unused GraphicsContext::isCompatibleBuffer()

No clients.

R=senorblanco@chromium.org,schenney@chromium.org

Review URL: https://codereview.chromium.org/70673002

git-svn-id: svn://svn.chromium.org/blink/trunk@161847 bbb929c8-8fbe-4397-9dbb-9b2b20218538