[omnibox] Add about:flag for WebUI omnibox popup

This CL just adds the flag for the WebUI Omnibox Popup to about:flags
a.k.a. chrome://flags.

This also fixes a trivial crash when the flag is on, as we haven't
paid much engineering effort to this flag recently.

I fixed the crash with this too, as otherwise, users who turn this flag
on won't be able to easily access chrome://flags to turn it off!

Bug: 1046561
Change-Id: Ie458c14b3e56d647d413d57325a26653a906e858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463747
Commit-Queue: Tommy Li <[email protected]>
Reviewed-by: Justin Donnelly <[email protected]>
Cr-Commit-Position: refs/heads/master@{#815846}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index e21a1148..db42c69 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -973,6 +973,9 @@
 extern const char kOmniboxUISwapTitleAndUrlName[];
 extern const char kOmniboxUISwapTitleAndUrlDescription[];
 
+extern const char kOmniboxWebUIOmniboxPopupName[];
+extern const char kOmniboxWebUIOmniboxPopupDescription[];
+
 extern const char kOnTheFlyMhtmlHashComputationName[];
 extern const char kOnTheFlyMhtmlHashComputationDescription[];