Inline the one call site of RenderLayer::clipParent.

Also, make clippedByLayerNotAboveCompositedAncestor a static
inline method in CompositedLayerMapping.cpp, since it is static
and the only callers are in that file.

BUG=

Review URL: https://codereview.chromium.org/347283005

git-svn-id: svn://svn.chromium.org/blink/trunk@177013 bbb929c8-8fbe-4397-9dbb-9b2b20218538