DevTools: validate remote front-end URLs

- validate a remote front-end URL before fetching it;
- only expose bindings in window if opener has bindings or there's
    no opener.

Bug: 662859
Change-Id: I3a5619b78dbd29dc730f37d704a212ecad8bbb54
Reviewed-on: https://chromium-review.googlesource.com/770511
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516756}
3 files changed