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The previous way we counted async match changes in
AutocompleteController was wrong, because I misunderstood how
AutocompleteResult::CopyOldMatches works.

Previously, CopyOldMatches would actually *move* the matches from the
old result to the new result. This operation cleared out
|last_result|. Since we determine if a match position has changed by
comparing |last_result| to the new |result_|, this overinflated the
number of changed matches.

I've verified this by adding detailed logging on my test machine.

This CL makes CopyOldMatches take a const parameter, and copy over the
old matches without mutating |last_result|. I think there's no way for
us to do this type of logging while still doing the destructive-move
from |last_result|, and I suspect doing a copy instead of a move will
have a negligible performance impact, and no memory impact.

Bug: 398135
Change-Id: I0e1c5f58c6264b5be4c3d12366ae6e3c66f2a04f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899314Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Kevin Bailey <krb@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714011}