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Creates and populates new UKM PreviewsDeferAllScript event to report
the count of scripts force deferred in the mainframe.

Bug: 978568
Change-Id: I7e7d87d0a181559832e1a4c6a1682db543790434
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715109Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680567}