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This implements setting and using the |headers| data in
LoadUrlParams. A new test fixture is added to ensure the extra headers
are properly received by the test server.

Bug: 923598
Change-Id: I59a44c886f1853bbc631ae016b0655dd1d4d539b
Reviewed-on: https://chromium-review.googlesource.com/c/1427565
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625912}