Inline some trivial functions in CompositingReasonFinder

This CL also moves CompositingReasonOverflowScrollingTouch inside
hasOverflowScrollTrigger because needsCompositedScrolling cannot return true if
we don't have the overflow scroll trigger.

Review URL: https://codereview.chromium.org/309593003

git-svn-id: svn://svn.chromium.org/blink/trunk@175206 bbb929c8-8fbe-4397-9dbb-9b2b20218538