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When search engine has valid prepopulate_id, DefaultSearchManager will
replace it with the entry from prepopulated list. This is not desired
behavior for Play API search engines as it will reset
created_from_play_api flag and search engine name and URLs.

This CL prevents DefaultSearchManager from modifying Play API search
engine.

BUG=1048713
R=pkasting@chromium.org,krb@chromium.org

Change-Id: I5e9a5e523f419b8a28953ce8f26003c1d207a451
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037830Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739180}