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This adds the StreamAnalyzer class, which is responsible
for calculating the mean, rms, smr, standard deviation,
variance of roots, and threshold percentiles of a
continuous stream of values.

It owns a Histogram and a WindowedAnalyzer, which it uses
to delegate percentile estimates and to track regions of
time where metrics are worst.

Bug: 807463
Change-Id: I718b06778582d0628b964747f61352ae291452f0
Reviewed-on: https://chromium-review.googlesource.com/972568
Commit-Queue: Brian Anderson <brianderson@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546168}