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The metric was split out of the WebView version, so we should use this
now for WebLayer startup tests.

Pinpoint job running:
https://pinpoint-dot-chromeperf.appspot.com/job/104b30a9520000

Bug: 1146438
Change-Id: I9938614b94740a3222fb832ab994427b136bf1d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536797Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827339}