Clean up and reference-ify SharedStyleFinder

Make almost all of SharedStyleFinder use references. This makes it more
clear what could be null. I also renamed and refactored a few simple
things like renaming Element::hasFlagsSetDuringStylingOfChildren to
childrenSupportStyleSharing (and negating the condition) to make it
better reflect what it does and made the ElementResolveContext into
a member variable instead of passing it down through every function
call.

Review URL: https://codereview.chromium.org/28083002

git-svn-id: svn://svn.chromium.org/blink/trunk@159972 bbb929c8-8fbe-4397-9dbb-9b2b20218538