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This uses a base::SingleSamplMetric to log the peak number of
instances per RenderFrame. We'll use this value to establish a
limit on the maximum allowed.

R=sandersd

Bug: 1144736
Change-Id: Ic0dac59d8682da0ea3185dcf97e7273bd9e037c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587493
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836917}