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It is only used in ios/components/io_thread/, so move it there.

Change-Id: Icc86011f711f2325a7bd33931fd8b12d6e2164ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373366Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820838}