Disallow squashing for layers with reflections

We're not smart enough to draw reflections into squashing layers. Instead,
these layers need to be separately composited so we can set up the reflection
layer correctly.

BUG=391022
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/367163003

git-svn-id: svn://svn.chromium.org/blink/trunk@177432 bbb929c8-8fbe-4397-9dbb-9b2b20218538