Hide DevTools frontend from webRequest API

Prevent extensions from observing requests for remote DevTools frontends
and add regression tests.

And update ExtensionTestApi to support initializing the embedded test
server and port from SetUpCommandLine (before SetUpOnMainThread).

BUG=797497,797500
TEST=browser_test --gtest_filter=DevToolsFrontendInWebRequestApiTest.HiddenRequests

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ic8f44b5771f2d5796f8c3de128f0a7ab88a77735
Reviewed-on: https://chromium-review.googlesource.com/844316
Commit-Queue: Rob Wu <rob@robwu.nl>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528187}
 AppViewGuestDelegate* ChromeExtensionsAPIClient::CreateAppViewGuestDelegate()
     const {
   return new ChromeAppViewGuestDelegate();
}

 AppViewGuestDelegate* ChromeExtensionsAPIClient::CreateAppViewGuestDelegate()
     const {
   return new ChromeAppViewGuestDelegate();
}
