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This CL adds a URLLoaderThrottle to renderer subresource requests
that delays the navigation by a fixed amount (default 50ms) if it
detects that the resource request is an insecure (e.g. http) ad
request.

The URLLoaderThrottle will be re-used in the browser process for
subframe navigations, so care has been made to fit it in the
content/common layer.

Some other misc changes to make this happen / testable:
 - Add IsAdResource() to blink::WebURLRequest
 - URLLoaderThrottleProvider changed to take a full blink::WebURLRequest
   instead of a blink::WebURL.
 - TestURLLoaderFactory extended to support redirects.
 - ThrottlingURLLoader partly exposed to content/public/test.

Bug: 829042
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ia98891ed8c07a3c85e1f9986c29d841698ed9228
Reviewed-on: https://chromium-review.googlesource.com/994152
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549300}