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This CL does the initial bringup of ad tagging and blocking
functionality in WebLayer. To do so, we do the following:
- Bring up an implementation of SubresourceFilterClient and attach it
  to WebContents instances
- Instantiate subresource filter navigation throttles on navigation
- Instantiates and configures SubresourceFilterAgent instances in the
  renderer

The organization and code added is modeled after that in //chrome. We
have left TODOs for significant pieces of functionality to be added
in followups (e.g., showing of the infobar on Android). One thing to
note is that ad tagging/blocking only triggers on Android in WebLayer,
as it requires having a safe browsing database instance and WebLayer
does not have one on platforms other than Android.

This CL also adds browsertests of this initial functionality:
- that the subresource filter activation state is computed on page loads
- that ads are blocked on activated pages if they are flagged by the
  ruleset

Bug: 1116095
Change-Id: I1db0136709014004c221bb0251551eaeac3538e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526087
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827735}