Move squash preventing reasons out of compositing reasons
This is a fairly mechanical change. I've replaced all of
the squashing-related compositing reasons a with a single
reason. I've also added squashing disallowed reasons to
capture the information we lost in the melding of
compositing reasons.
This caused a number of tests to bark, so I've patched
them up. I've also ensured that the squashing disallowed
reasons are plumbed out through the graphics layer debug
info so that they still show up via about:tracing.
BUG=589495
Review URL: https://codereview.chromium.org/1730653002
Cr-Commit-Position: refs/heads/master@{#377398}
17 files changed
