[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}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index f695908..46451aa 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -870,6 +870,9 @@
 extern const char kOmniboxOnDeviceHeadSuggestionsName[];
 extern const char kOmniboxOnDeviceHeadSuggestionsDescription[];
 
+extern const char kOmniboxPreserveDefaultMatchAgainstAsyncUpdateName[];
+extern const char kOmniboxPreserveDefaultMatchAgainstAsyncUpdateDescription[];
+
 extern const char kOmniboxUIShowPlaceholderWhenCaretShowingName[];
 extern const char kOmniboxUIShowPlaceholderWhenCaretShowingDescription[];