Rename RenderStyle::diff and RenderStyle::compare to better names

Having both ::compare() and ::diff() was really confusing, this patch
renames them to stylePropagationDiff() and visualInvalidationDiff()
making their purpose more clear.

Review URL: https://codereview.chromium.org/214473004

git-svn-id: svn://svn.chromium.org/blink/trunk@170223 bbb929c8-8fbe-4397-9dbb-9b2b20218538