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Previously, the HTTP Auth Cache was implemented as a list of auth
entries, which was searched linearly for a matching entry upon lookup.
This change converts the cache to a multimap keyed on the origin, which
is expected to provide faster lookups in cases where there are no
matching entries for a given origin (which is the majority of cases).
This will also allow for a higher cache size limit without a linear
increase in lookup time, hence the cache size limit is raised from 10 to
20. This is expected to lower the rate of evictions upon adding entries.
Obsolete histograms which no longer apply to the new implementation are
also removed.

Bug: 757116, 614108
Change-Id: Iabd2104b2d1c38d474450978a712039819b038e6
Reviewed-on: https://chromium-review.googlesource.com/c/1273820
Commit-Queue: Lily Chen <chlily@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599362}