Cleanup: Delete WebSolidColorLayer API.

Chromium stopped overridding createSolidColorLayer() from
WebCompositorSupport and inheriting from WebSolidColorLayer at
https://src.chromium.org/viewvc/chrome?revision=288125&view=revision, and
Blink removed the function at
https://src.chromium.org/viewvc/blink?revision=179785&view=revision.

BUG=None
TEST=None
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/460453003

git-svn-id: svn://svn.chromium.org/blink/trunk@179892 bbb929c8-8fbe-4397-9dbb-9b2b20218538