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Adds test fixture and some simple tests for BlinkUrlLoader. This class
is still being implemented, so these tests mainly serve as examples.

Also implements BlinkUrlLoader::GrantUniversalAccess(), to make the
tests a little more interesting.

BlinkUrlLoader::Client was changed to provide a WebAssociatedURLLoader
instead of a WebLocalFrame. The former is easier to fake in unit tests.

Bug: 1099022
Change-Id: I66a8132373cf61ab539321151253a20bbf7801b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391928
Commit-Queue: K. Moon <kmoon@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804292}