Reapply 169354 "DevTools: track stylesheets by document, not local frame"

BUG=353257
TBR=vsevik

Review URL: https://codereview.chromium.org/203043003

git-svn-id: svn://svn.chromium.org/blink/trunk@169421 bbb929c8-8fbe-4397-9dbb-9b2b20218538