Use LocalStyleChange for classes in rightmost compound selector.

If we resort to not using invalidation sets, we can still use
LocalStyleChange instead of SubtreeStyleChange for classes in the rightmost
compound selector if the invalidation set approach was not dropped due to a
feature in the rightmost compound selector. For classes in other compound
selectors, still use SubtreeStyleChange.

Examples:

".a + .b" -> LocalStyleChange for "b", SubtreeStyleChange for "a"
".a .b::first-letter" -> SubtreeStyleChange for both "a" and "b".

R=esprehn@chromium.org,chrishtr@chromium.org
BUG=335247

Review URL: https://codereview.chromium.org/199373003

git-svn-id: svn://svn.chromium.org/blink/trunk@169255 bbb929c8-8fbe-4397-9dbb-9b2b20218538