StyleEngine: Encapsulate dirty scope tracking

This change extracts StyleTreeScopeTracker from StyleEngine.
The class encapsulates the special-case handling for
document scope to improve readability on StyleEngine side.

BUG=none
R=esprehn, tasak, dglazkov
TEST=none

Review URL: https://codereview.chromium.org/77963003

git-svn-id: svn://svn.chromium.org/blink/trunk@162439 bbb929c8-8fbe-4397-9dbb-9b2b20218538