Inline RenderLayerCompositor::layerWasAdded

This function ignores both its arguments and is one line. It's clearer if
we just inline it and match it up with the call to
setNeedsCompositingInputsUpdate.

Review URL: https://codereview.chromium.org/326253002

git-svn-id: svn://svn.chromium.org/blink/trunk@175938 bbb929c8-8fbe-4397-9dbb-9b2b20218538