Split out the style-determined direct compositing reasons

This CL splits the direct compositing reasons into two groups: those that are
determined from style information alone and those that require some non-style
information. This CL simply separates the two groups. A future CL will move the
computation of the style-determined reasons to recalc style.

Review URL: https://codereview.chromium.org/206413009

git-svn-id: svn://svn.chromium.org/blink/trunk@169715 bbb929c8-8fbe-4397-9dbb-9b2b20218538