--disable-layer-squashing should disable layer squashing

To avoid complicated three-sided CLs to enable layer squashing, we hard-enabled
squashing yesterday. Now that all the repos have caught up to each other, we
can connect --disable-layer-squashing to actually control whether layer
squashing is enabled.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/245563007

git-svn-id: svn://svn.chromium.org/blink/trunk@172085 bbb929c8-8fbe-4397-9dbb-9b2b20218538