updateSquashingLayers can fail to create a squashing container

updateSquashingLayers needs to ensure that there's either an
m_ancestorClippingLayer or a m_squashingContainmentLayer (but not both).
Previously, this code wouldn't work properly if m_ancestorClippingLayer was
created or destroyed by updateClippingLayers, which caused full-screen videos
on YouTube to crash.

R=vollick@chromium.org
BUG=365579

Review URL: https://codereview.chromium.org/246623007

git-svn-id: svn://svn.chromium.org/blink/trunk@172164 bbb929c8-8fbe-4397-9dbb-9b2b20218538