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This CL adds tests that verify that Cross-Origin Read Blocking (CORB)
won't block HTTP responses for requests initiated by plugins that have
been granted universal access (like Flash which has its own CORS-like
mechanism - crossdomain.xml).

In addition to running tryjobs, I've also tested this CL by temporarily
editing CrossSiteDocumentResourceHandler::ShouldBlockBasedOnHeaders to
comment out the exception carved out for plugins (the new
TestTrustedCorbEligibleRequest test expectedly failed with such an edit
both with and without NetworkService).

Bug: 846339
Change-Id: I7b7befff3018ed5a7bd834b2bf0507c2e1f6365b
Reviewed-on: https://chromium-review.googlesource.com/1101300
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568081}