DevTools: defer styles delta calculation to until the end of the task.

BUG=289708
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/201363002

git-svn-id: svn://svn.chromium.org/blink/trunk@169371 bbb929c8-8fbe-4397-9dbb-9b2b20218538