Re-enable solid background color optimization for composited layers.

This has two parts:
1.) Make GraphicsLayer::setContentsToSolidColor() handle a solid color layer.
2.) Remove GraphicsLayer::supportsBackgroundColorContent() because we have only
one compositor implementation.

BUG=114658, 232846
TEST=compositing/background-color/background-color-change-to-text.html
compositing/background-color/background-color-change-to-transparent.html
compositing/background-color/*

Review URL: https://codereview.chromium.org/63943006

git-svn-id: svn://svn.chromium.org/blink/trunk@162737 bbb929c8-8fbe-4397-9dbb-9b2b20218538