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This CL adds UKM to count the number of async scripts associated with
a given mainframe document.

Google-internal UKM privacy review:
https://docs.google.com/document/d/1bKyDRF5BAQpPJ-bRyKs2dgqBTJI43b63kOULO-HIuXM/edit

Bug: 1086227
Change-Id: Iaaee8c756952613c2ea58416b453a98a0843773f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209118
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771611}