Cache pointers to histograms for recording event times.
Cache pointers to histograms for recording event times.

Doing a FactoryGet for every UI event (including every
mouse-move event) is expensive and unnecessary.  This
will reduce the time spent updating the histogram from
about 5us to about 0.8us (measured on Z620).

A reusable macro is added to allow any enumerated block
of histograms to be cached.

BUG=597011

Review URL: https://codereview.chromium.org/1829973002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386086}

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