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This patch adds some browser tests that verifies CORS checks for
requests initiated from local files.

Tests verify if;
 - HTTP request has an expected Origin header, 'null' for production mode,
   'file://' for test mode with --allow-file-access-from-files flag.
 - CORS check pass iff the server allow 'null' origin for production
    mode, 'file://' origin for the test mode.

These tests should be provided over file:// protocol, and need to modify
--allow-file-access-from-files flag. That requires exaggerated setup if
we want to have these tests in Blink layout tests, that's why these are
in browser tests here though relevant implementations are in Blink and
the network service.

Bug: 825567
Change-Id: I986a110f6cddc0349b83cb0ade16ef30581c6ce4
Reviewed-on: https://chromium-review.googlesource.com/987593Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549447}