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DatabaseConnections::connections_ is only mutable so that
DatabaseConnections::GetOpenDatabaseSize() can use std::map::operator[]
while being const.

This CL removes the mutable qualifier and replaces operator[] use in
GetOpenDatabaseSize() with find() and const_iterator, to make it easier
to reason about thread safety and invariants.

Change-Id: Ifb9df3733c099d9b686096f71156d0a4b2c6271a
Reviewed-on: https://chromium-review.googlesource.com/1137929Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575454}