Put HashTable::checkKey() in place

This is the partial revert of r155808.

We need to keep 'checkKey()' to catch the possible HashTable misuse
(if empty or deleted keys are attempted to pass) and to make it
clear that such keys are not expected.

The 'checkKey()' calls are not put back to 'lookupForWriting'
and 'fullLookupForWriting' methods since the passed keys are
already checked within the parent calls.

Review URL: https://codereview.chromium.org/216153004

git-svn-id: svn://svn.chromium.org/blink/trunk@170775 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix squashing layers with different clipping containers.

Layers may not squash unless they share the same clipping
container as the squashing CLM owner. Previously the
implementation of this had two bugs: (a) it traversed in
paint order, not tree order, and kept a running clipping
ancestor along the way, and (b) it did not use the same
logic as the CompositedLayerMapping did for allocating an
m_ancestorClippingLayer for a squashing CLM.

BUG=358507

Review URL: https://codereview.chromium.org/222863002

git-svn-id: svn://svn.chromium.org/blink/trunk@170776 bbb929c8-8fbe-4397-9dbb-9b2b20218538