Remove hasContentsLayer and isMainFrameRenderViewLayer from CLM

This patch removes some deadcode from CompositedLayerMapping:
1) hasContentsLayer - this function name was confusing.
2) isMainFrameRenderViewLayer - state is now all within CLM.

No new tests, no change in behavior.

Review URL: https://codereview.chromium.org/542863007

git-svn-id: svn://svn.chromium.org/blink/trunk@181526 bbb929c8-8fbe-4397-9dbb-9b2b20218538