[Extensions] Restrict debugging file:-scheme URLs

Don't allow extensions to debug file:-scheme URLs if the extension does
not have explicit file access (as set in chrome://extensions). Achieve
this by introducing a new virtual method on DevToolsAgentHostClient to
allow the implementor to check if a given host is allowed to be
inspected.

Add regression tests for the same.

Bug: 666299

Change-Id: Icb5ee89bf788643eee166eef83802d10ab825a6c
Reviewed-on: https://chromium-review.googlesource.com/1104954
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569828}
12 files changed