Lock histograms_ object when fetching begin/end iterators.
Lock histograms_ object when fetching begin/end iterators.

BUG=604506

Review URL: https://codereview.chromium.org/1900113002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388251}

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