Have NodeRareData's ensureNodeLists() / ensureMutationObserverData() return a reference

Have NodeRareData's ensureNodeLists() / ensureMutationObserverData() return a reference
instead of a pointer as the return value can never be null. This makes the calling
code look safer.

R=adamk
BUG=

Review URL: https://codereview.chromium.org/26852004

git-svn-id: svn://svn.chromium.org/blink/trunk@160213 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Partially revert "Reduce the number of hash lookups in requiresCompositingForAnimation"

This partially reverts r155284, which made opacity animations require
compositing even when we weren't already in compositing mode,
triggering an underlying bug with CSS transitions in a composited
iframe.

BUG=308851

Review URL: https://codereview.chromium.org/29083003

git-svn-id: svn://svn.chromium.org/blink/trunk@160214 bbb929c8-8fbe-4397-9dbb-9b2b20218538