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This is a step toward supporting Slow Page Triggering as may be indicated
in the optmization guide hints. A follow-on CL will use this surfaced
ECT threshold.

Bug: 895581
Change-Id: Ic8cca4c03eb1700223a83980e9376e7612197290
Reviewed-on: https://chromium-review.googlesource.com/c/1289150Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600934}