GitLab

- Merges the new header info with the old one in ACResult rather than
  replacing it. The old header info may be needed by the old matches.
- Clears the header info along with matches in AutocompleteResult::Reset
- Moves the logic to strip the match group IDs that don't have an
  equivalent header string next to the logic for grouping and demoting
  matches with headers in ACResult for improved readability.  

Bug: 1122669, 1130826, 1130880
Change-Id: Ibddaa30ccd19969a71e10befa993adb688a1f572
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424549Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810444}