Improve the compositing reasons when squashing fails

Currently we just report that "no squashing target was found". There
are many reasons why squashing might fail and this message is not very
informative. This cl expands the list of squashing-related compositing
reasons to make diagnosing layer explosions easier.

R=chrishtr@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/331203003

git-svn-id: svn://svn.chromium.org/blink/trunk@176259 bbb929c8-8fbe-4397-9dbb-9b2b20218538