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The creation will happen on |task_runner_| when |model_update_listener|
fires the new model available notification, instead of synchronously
on receiving the user autocomplete input.

This is to fix the omnibox.QueryTime2.1 regression (other QueryTime2.x
is not affected by the existing design), e.g.
http://screenshot/1U2Oq5vfnRm

Tested on Pixel 2 XL for pre-existing model loading and new model
download.

Bug: 925072
Change-Id: Ieb26aa78c8bbd848ed1f0c9b722bab266b21aa59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902513
Commit-Queue: Ce Chen <cch@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713978}