Set up a field trial flag to prerender Instant search base url on omnibox focus event.

chrome/browser/search/search.h

Index: chrome/browser/search/search.h
diff --git a/chrome/browser/search/search.h b/chrome/browser/search/search.h
index 3cbd7d8c01d59765f49ef27c7e2253056c37bcae..75c4aeb5a25f98a06f72b60957fe475b6f7b5668 100644
--- a/chrome/browser/search/search.h
+++ b/chrome/browser/search/search.h
@@ -166,6 +166,11 @@ GURL GetSearchResultPrefetchBaseURL(Profile* profile);
 // prefetch high-confidence search suggestions.
 bool ShouldPrefetchSearchResults();
 
+// Returns true if 'prerender_instant_url_on_omnibox_focus' flag is enabled in
+// field trials to prerender Instant search base page when the omnibox is
+// focused.
+bool ShouldPrerenderInstantUrlOnOmniboxFocus();
+
 // Returns true if 'reuse_instant_search_base_page' flag is set to true in field
 // trials to reuse the prerendered page to commit any search query.
 bool ShouldReuseInstantSearchBasePage();