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This adds a test for clearing the bad proxy cache when DRP is turned
off, and implements that functionality with network service.

Bug: 721403
Change-Id: I12f441191de6dfb890c7a042b72a5a87f72e41aa
Reviewed-on: https://chromium-review.googlesource.com/c/1343352Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609846}