Add Apps.AppListSearchQueryLength UMA histogram.

ui/app_list/app_list_constants.cc

Index: ui/app_list/app_list_constants.cc
diff --git a/ui/app_list/app_list_constants.cc b/ui/app_list/app_list_constants.cc
index 29b4a9b2eb17637b12c35cc9107ab001d620f563..a3c0ada4bce6a28e5b9723ef8fc23bbaa423ac03 100644
--- a/ui/app_list/app_list_constants.cc
+++ b/ui/app_list/app_list_constants.cc
@@ -129,6 +129,10 @@ const char kPageOpenedHistogram[] = "Apps.AppListPageOpened";
 const char kSearchResultOpenDisplayTypeHistogram[] =
     "Apps.AppListSearchResultOpenDisplayType";
 
+// The UMA histogram that logs how long the search query was when a result was
+// opened.
+const char kSearchQueryLength[] = "Apps.AppListSearchQueryLength";
+
 #if defined(OS_LINUX)
 #if defined(GOOGLE_CHROME_BUILD)
 const char kAppListWMClass[] = "chrome_app_list";