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It allows us to observe mainframe download in PageLoadMetricsObserver.

Bug: 881345
Change-Id: Icc5a3fb9dd645b9bdc291b64030789312614b74d
Reviewed-on: https://chromium-review.googlesource.com/c/1260566Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597576}