Make WebFrame::isLoading() local-only and map it to load event completion

Also, add isResourceLoadInProgress() for use in content shell, to determine
when to complete tests.

Exposing this information will allow content shell to better
evaluate when to end tests, hopefully eventually leading
blink being able to define load completion strictly as
the completion of the load event.

BUG=

Review URL: https://codereview.chromium.org/514603004

git-svn-id: svn://svn.chromium.org/blink/trunk@181570 bbb929c8-8fbe-4397-9dbb-9b2b20218538