Have Node::highestAncestor() return a reference

Have Node::highestAncestor() return a reference instead of a pointer as it can
never return null. This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/182053002

git-svn-id: svn://svn.chromium.org/blink/trunk@167964 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Compositing update calls RenderLayerStackingNode::isStackingContext too much

This CL simplifies isStackingContext and makes RenderLayer call
isStackingContext fewer times. This function is hot on the compositing update
profile for toggle_drawer in the Silk page set. After this change, I didn't see
a big change to the profile, but it's silly to do all this work twice.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/180843006

git-svn-id: svn://svn.chromium.org/blink/trunk@167965 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix bug in which not all style invalidation dirty bits were being cleared
when whole-subtree invalidation was triggered.

BUG=346928

Review URL: https://codereview.chromium.org/181043004

git-svn-id: svn://svn.chromium.org/blink/trunk@167966 bbb929c8-8fbe-4397-9dbb-9b2b20218538