Ensure that pointers in CompositedLayerMapping::m_squashedLayers are not stale.

Squashing is still disabled on ToT, but some layout tests force the feature on.
Presently, RenderLayers can be destroyed before their groupedMapping, causing
dangling pointers in CompositedLayerMapping. This fixes the issue by adding some
cleanup code in ~RenderLayer().

BUG=340687

Review URL: https://codereview.chromium.org/149023009

git-svn-id: svn://svn.chromium.org/blink/trunk@166763 bbb929c8-8fbe-4397-9dbb-9b2b20218538