WebUI Tab Strip: Add metrics for New Tab button inside the tab strip

Bug: 1032778
Change-Id: I7701cdfe36f29110d1da957d87605ddbbc8b9179
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013912Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734129}

chrome/browser/ui/tabs/tab_strip_model.h

@@ -131,8 +131,12 @@ class TabStripModel : public TabGroupController {
     // WebUI touch-optimized tab strip.
     NEW_TAB_BUTTON_IN_TOOLBAR_FOR_TOUCH = 3,
 
+    // New tab was opened through the new tab button inside of the WebUI tab
+    // strip.
+    NEW_TAB_BUTTON_IN_WEBUI_TAB_STRIP = 4,
+
     // Number of enum entries, used for UMA histogram reporting macros.
-    NEW_TAB_ENUM_COUNT = 4,
+    NEW_TAB_ENUM_COUNT = 5,
   };
 
   static constexpr int kNoTab = -1;

chrome/browser/ui/webui/tab_strip/tab_strip_ui_handler.cc

@@ -411,6 +411,9 @@ void TabStripUIHandler::RegisterMessages() {
 }
 
 void TabStripUIHandler::HandleCreateNewTab(const base::ListValue* args) {
+  UMA_HISTOGRAM_ENUMERATION("Tab.NewTab",
+                            TabStripModel::NEW_TAB_BUTTON_IN_WEBUI_TAB_STRIP,
+                            TabStripModel::NEW_TAB_ENUM_COUNT);
   chrome::ExecuteCommand(browser_, IDC_NEW_TAB);
 }