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This is a re-land of:
https://codereview.chromium.org/2435813002/

The reland keeps a TimesHistogramSample class in LaunchMetrics
since it's still referenced in internal code. Once this lands, the
clank/ code can be updated and this class can be cleaned up.

Original CL description:

This is general-purpose functionality that allows
recording UMA metrics before the C++ native library
has been loaded and is useful to have outside of the
Chrome layer. In particular, I will be using it from
components/variations to record information about the
first run variations seed fetch.

No functional changes in this CL.

BUG=632199
TBR=bauerb@chromium.org, thestig@chromium.org

Review-Url: https://chromiumcodereview.appspot.com/2439113002
Cr-Commit-Position: refs/heads/master@{#426853}