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This is a followup of https://codereview.chromium.org/2975623002/.
It prevents the renderer from ignoring main resource requests with the "cid"
scheme and adds tests that check an Android Webview can intercept any requests
with this scheme.

BUG=739658

Change-Id: I03e039f5e84f2be8638c46812f2216803a0f1d4d
Reviewed-on: https://chromium-review.googlesource.com/574591Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488330}