Fix error recovery in @-webkit-region body.

Currently CSS parser stops parsing everything inside @-webkit-region body
once it find something which is not a valid_region_rule. It could be a rule
with an invalid selector. The same happens with @-rules not allowed in
its body (@import, for example). However it ignores syntactically incorrect
allowed @-rules (for instance, "@font_face;").

This change makes the parser ignore all incorrect rules inside
@-webkit-region blocks.

TBR=tabatkins@gmail.com

Review URL: https://codereview.chromium.org/78553002

git-svn-id: svn://svn.chromium.org/blink/trunk@162389 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Separate compositedLayerMapping allocation from computing compositing requirements.

For layer squashing, we need to have a separate phase of compositing
update that walks the tree and assigns how each layer will be handled
(i.e. have a separate backing, no backing, or squashed into a grouped
backing). This patch creates that new function, assignLayersToBackings.

BUG=261605

Review URL: https://codereview.chromium.org/75653004

git-svn-id: svn://svn.chromium.org/blink/trunk@162390 bbb929c8-8fbe-4397-9dbb-9b2b20218538