Lock histograms during iteration.
Lock histograms during iteration.

Mac builds are crashing due to invalid dereferences with the only
explanation I can see being a race-condition of iterating over
the tree while it is being added to on another thread.

The alternative would be a larger change that goes back to fetching
a vector of histograms (GetHistograms) in one shot within the calling
code.

I've left the NULL in (like the rest of the file) rather than
change it to nullptr because this is just a quick bug-fix.

BUG=583287

Review URL: https://codereview.chromium.org/1659913002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372981}

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