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The Declarative Net Request Page Whitelisting API uses initiator checks to guess
the main frame url for a main-frame subresource request. This CL adds UMA to
enumerate how often the different cases occur. This should help reason about the
correctness of these checks.

BUG=811460

Change-Id: Id72c05041477d05825fbe7e0ff7fecbf39fda861
Reviewed-on: https://chromium-review.googlesource.com/1089475Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565473}