Reapply 169371 "DevTools: defer styles delta calculation to until the end of the task"

BUG=353257
TBR=vsevik

Review URL: https://codereview.chromium.org/202893003

git-svn-id: svn://svn.chromium.org/blink/trunk@169422 bbb929c8-8fbe-4397-9dbb-9b2b20218538