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It looks like some of the changes to ThrottlingURLLoader in
http://crrev.com/c/1157549 caused some perf regressions on mobile. This
fixes the regressions. Something with how StartInfo was created and
passed around before was causing problems.

See bugs for more info. Confirmed regressions fixed with this CL using
pinpoint.

Bug: 874902, 873881, 874918
Change-Id: I34397b8f97cffa82857f87ed19ac6ef5b61a7cac
Reviewed-on: https://chromium-review.googlesource.com/1178576Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584086}