Remove more unused method declaration from RenderLayerCompositor.h

These methods are declared, but not used.

BUG=None

Review URL: https://codereview.chromium.org/536173002

git-svn-id: svn://svn.chromium.org/blink/trunk@181351 bbb929c8-8fbe-4397-9dbb-9b2b20218538