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This will call WebMeasureMemory randomly after a page load, and discard
the results, to check for crashes. It is controlled by an experiment
feature ("StressTestWebMeasureMemory") that will only be enabled for
some Canary users.

Also removes some unused headers from performance_manager_lifetime.cc.

Bug: 1085129
Change-Id: I32f24d4bf35032dea26b672a4fa899c5bf182cf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627360Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843099}