Factor CompositingReasonFinder out of RenderLayerCompositor

RenderLayerCompositor is a large block of code. This CL moves a chunk of code
related to finding the direct compositing reasons into its own class. This code
doesn't interact much with the rest of RenderLayerCompositor.

Eventually, we might want to update the direct compositing reasons during style
recalc (rather than during compositing update). Having the code in a separate
class will make that easier.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/178743002

git-svn-id: svn://svn.chromium.org/blink/trunk@167726 bbb929c8-8fbe-4397-9dbb-9b2b20218538