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This CL contains the implementation a for the flag to preserve the
default match during the async pass.

It's a minimally-interventionist approach (the smallest possible
hammer) that simply promotes the previous default match to the top, if
it can be found in the new result set from the async pass.

It won't inject in the old default match if it's truly no longer
present after the async pass, because I think there will be unwanted
side effects from that.

It also adds a test.

Bug: 398135
Change-Id: I9be0f18820436d3d29b8ba7a80835d456d771cc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1788313Reviewed-by: Kevin Bailey <krb@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695324}