[omnibox] Add feature flag for using web content to render omnibox popup

Just a feature flag for investigatory work. No concrete plans yet.

Bug: 1046561
Change-Id: I328f4d8e7edd2a4eee012202f7363a3e7374ee35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026123Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737428}

components/omnibox/common/omnibox_features.cc

@@ -331,6 +331,11 @@ const base::Feature kOmniboxSuggestionButtonRow{
 const base::Feature kTabKeyCanEscapeOmniboxPopup{
     "TabKeyCanEscapeOmniboxPopup", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// If enabled, uses WebUI to render the omnibox suggestions popup, similar to
+// how the NTP "fakebox" is implemented.
+const base::Feature kWebUIOmniboxPopup{"WebUIOmniboxPopup",
+                                       base::FEATURE_DISABLED_BY_DEFAULT};
+
 // When enabled, use Assistant for omnibox voice query recognition instead of
 // Android's built-in voice recognition service. Only works on Android.
 const base::Feature kOmniboxAssistantVoiceSearch{

components/omnibox/common/omnibox_features.h

@@ -68,6 +68,7 @@ extern const base::Feature kConfirmOmniboxSuggestionRemovals;
 extern const base::Feature kOmniboxLooseMaxLimitOnDedicatedRows;
 extern const base::Feature kOmniboxSuggestionButtonRow;
 extern const base::Feature kTabKeyCanEscapeOmniboxPopup;
+extern const base::Feature kWebUIOmniboxPopup;
 
 // Omnibox UI - these affect the UI or function of the location bar (not the
 // popup).