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This CL deletes WebRequestEventRouterDelegate and its implementation
ChromeExtensionWebRequestEventRouterDelegate. It's only method,
NotifyWebRequestWithheld is replaced with a method on the ExtensionsAPIClient.
This will make it easier to share the implementation with the Declarative Net
Request API.

This CL should have no behavior change.

BUG=809680

Change-Id: I696dbf8d3a34ae546342eb7ba98a1dd9e26fe3aa
Reviewed-on: https://chromium-review.googlesource.com/c/1256218
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595979}