Don't report hierarchy changes when mask layer changes

In https://codereview.chromium.org/239053008/, I'd made mask layer
changes affect layerConfigChanged so that I could use this value later
in the function to trigger a painting phase update. This causes
unnecessary work later in the pipeline as it causes the rest of the
machinery to think that the graphics layer tree topology has changed.

This CL tracks whether the mask layer has changed in a separate
variable so that we can still use it as a paint phase update trigger,
but not impact graphics layer tree updating.

R=abarth@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/258543004

git-svn-id: svn://svn.chromium.org/blink/trunk@172556 bbb929c8-8fbe-4397-9dbb-9b2b20218538