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This gives the core stability metrics.

BUG=1120537
TEST=covered by weblayer_browsertests

Change-Id: I22e99959dacb9b9593af02cea70088d449e604a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376565Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801836}