Make GraphicsLayer::contentsLayer() protected.

This method is exposed for tests, and very error prone because its name is too
similar to GraphicsLayer::contentLayer().
So this CL makes it protected.

Review URL: https://codereview.chromium.org/83463002

git-svn-id: svn://svn.chromium.org/blink/trunk@162587 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Unify GraphicsLayer::setContentsToMedia and setContentsToCanvas.

Merge setContentsToMedia and setContentsToCanvas into setContentsToPlatformLayer.

This CL is based on timothy_horton@apple.com's work: webkit.org/b/109658.

Review URL: https://codereview.chromium.org/83493002

git-svn-id: svn://svn.chromium.org/blink/trunk@162588 bbb929c8-8fbe-4397-9dbb-9b2b20218538