Fix bug in which state transitions from squashed -> individually composited were not computed correctly.
Also some small cleanup in requiresSquashing.

BUG=342630,315321

Review URL: https://codereview.chromium.org/138763005

git-svn-id: svn://svn.chromium.org/blink/trunk@167040 bbb929c8-8fbe-4397-9dbb-9b2b20218538