CompositedLayerMapping::updateInternalHierarchy crashes with squashing

Before this CL, CompositedLayerMapping::updateScrollingLayers
could call updateInternalHierarchy, but that's inconsistent
with the general approach of these functions, which is to
return a bool indicating whether the internal hierarchy has
changed. When updateScrollingLayers was called in the
CompositedLayerMapping destructor, we would update the
internal hierarchy while it was in an inconsistent state,
causing a crash.

In this CL, I've removed the call to updateInternalHierarchy
in updateScrollingLayers to prevent this inconsistent update.
Instead, I've taught the one caller of updateScrollingLayers
how to do the extra work that updateScrollingLayers requires
when it changes layers.

BUG=366281

Review URL: https://codereview.chromium.org/258453002

git-svn-id: svn://svn.chromium.org/blink/trunk@172529 bbb929c8-8fbe-4397-9dbb-9b2b20218538