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This CL extends expiry for a histogram that will still be useful for
experimentation in a few following milestones.

Bug: 1117116
Change-Id: I4722a1452a92ceb99669b83735b542b0225f8b8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2478602
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818101}