[NTP4] Track the position of each app that's launched.

chrome/browser/ui/webui/ntp/app_launcher_handler.cc

Index: chrome/browser/ui/webui/ntp/app_launcher_handler.cc
diff --git a/chrome/browser/ui/webui/ntp/app_launcher_handler.cc b/chrome/browser/ui/webui/ntp/app_launcher_handler.cc
index ef81c8e661ee337a839a066919c4764981c092f1..04ac81522435205a1e45db7f2a84b871ce72a6d1 100644
--- a/chrome/browser/ui/webui/ntp/app_launcher_handler.cc
+++ b/chrome/browser/ui/webui/ntp/app_launcher_handler.cc
@@ -68,6 +68,11 @@ extension_misc::AppLaunchBucket ParseLaunchSource(
   return bucket;
 }
 
+enum {
+  APP_LAUNCHED_FROM_INDEX_WIDE_OFFSET = 100,
+  APP_LAUNCHED_FROM_INDEX_BOUNDARY = 200,
+};
+
 }  // namespace
 
 AppLauncherHandler::AppInstallInfo::AppInstallInfo() {}
@@ -235,6 +240,9 @@ void AppLauncherHandler::RegisterMessages() {
   web_ui()->RegisterMessageCallback("recordAppLaunchByURL",
       base::Bind(&AppLauncherHandler::HandleRecordAppLaunchByURL,
                  base::Unretained(this)));
+  web_ui()->RegisterMessageCallback("recordAppLaunchedFromIndex",
+      base::Bind(&AppLauncherHandler::HandleRecordAppLaunchedFromIndex,
+                 base::Unretained(this)));
   web_ui()->RegisterMessageCallback("closeNotification",
       base::Bind(&AppLauncherHandler::HandleNotificationClose,
                  base::Unretained(this)));
@@ -769,6 +777,24 @@ void AppLauncherHandler::HandleRecordAppLaunchByURL(
   RecordAppLaunchByURL(Profile::FromWebUI(web_ui()), url, bucket);
 }
 
+void AppLauncherHandler::HandleRecordAppLaunchedFromIndex(
+    const base::ListValue* args) {
+  double index_double;
+  CHECK(args->GetDouble(0, &index_double));
+  bool wide;
+  CHECK(args->GetBoolean(1, &wide));
+
+  // +1 to make it a natural (i.e. 1,2,3 instead of 0,1,2).
+  size_t index = static_cast<size_t>(index_double) + 1;
+  if (wide)
+    index += APP_LAUNCHED_FROM_INDEX_WIDE_OFFSET;
+  else if (index >= APP_LAUNCHED_FROM_INDEX_WIDE_OFFSET + 1)
+    index = APP_LAUNCHED_FROM_INDEX_BOUNDARY + 1;
+
+  UMA_HISTOGRAM_ENUMERATION("NewTabPage.AppLaunchedFromIndex", index,
+                            APP_LAUNCHED_FROM_INDEX_BOUNDARY);
+}
+
 void AppLauncherHandler::HandleNotificationClose(const ListValue* args) {
   std::string extension_id;
   CHECK(args->GetString(0, &extension_id));