[omnibox] Add about:flag to preserve default match during async pass

This CL adds kOmniboxPreserveDefaultMatchAgainstAsyncUpdate to
about:flags.

Bug: 398135
Change-Id: I0b9b3e8c2737b69807d4ec910f7e06ce25e35c8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789963
Commit-Queue: Tommy Li <[email protected]>
Reviewed-by: manuk hovanesian <[email protected]>
Cr-Commit-Position: refs/heads/master@{#694469}
5 files changed