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This change adds a histogram that tracks the number of cache entries
examined when no matching entry is found (which is most of the time).
This will allow us to have a better proxy for HTTP auth cache size.

Bug: 757116
Change-Id: I8a4b7348a473d2802ac12dda2589569e523b3e5e
Reviewed-on: https://chromium-review.googlesource.com/c/1265435Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597314}