Simplify RenderLayer::scrollsWithRespectTo

This CL makes it clearer what RenderLayer::scrollsWithRespectTo is computing.
Also, this CL saves 15% of the time in the compositing update when moving the
drawer in the calculator.

BUG=387134

Review URL: https://codereview.chromium.org/329953004

git-svn-id: svn://svn.chromium.org/blink/trunk@176676 bbb929c8-8fbe-4397-9dbb-9b2b20218538