Move Node::diff into RenderStyle and break it into parts

Node::diff doesn't actually diff nodes, but instead compares RenderStyles. This patch
renames it RenderStyle::compare and breaks it out into several functions. It also
reworks the function to be easier to understand by grouping related comparisons
and using early returns.

Review URL: https://chromiumcodereview.appspot.com/23541007

git-svn-id: svn://svn.chromium.org/blink/trunk@157353 bbb929c8-8fbe-4397-9dbb-9b2b20218538