Take CompatibleImageBufferSurface out behind the woodshed.

There's no reason for it to exist, since it's always created with a
raster-backed surface anyway, so UnacceleratedImageBufferSurface will
do just fine.

Covered by existing tests.

BUG=

Review URL: https://codereview.chromium.org/579953002

git-svn-id: svn://svn.chromium.org/blink/trunk@182212 bbb929c8-8fbe-4397-9dbb-9b2b20218538