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This change introduces the ExtensionURLLoaderThrottle which
exists in the renderer and replaces ExtensionRequestLimitingThrottle
when the network service is both enabled and disabled.

Also re-enabled two extension throttle tests to speculatively
fix issue 836188. Hopefully now that the throttling is done in
the renderer they will no longer be flaky.

Bug: 784576,836188
Cq-Include-Trybots: luci.chromium.try:linux_mojo;master.tryserver.chromium.linux:linux_mojo
Change-Id: Ib0c18f7b3fb17f4fbe47d79bf2f5bd7f1ebd6e06
Reviewed-on: https://chromium-review.googlesource.com/914589Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577168}