Add perf histogram and tests for inline signin

chrome/browser/resources/gaia_auth_host/gaia_auth_host.js

Index: chrome/browser/resources/gaia_auth_host/gaia_auth_host.js
diff --git a/chrome/browser/resources/gaia_auth_host/gaia_auth_host.js b/chrome/browser/resources/gaia_auth_host/gaia_auth_host.js
index c77ea9b84eec5f6ad68f6424b7c869e2ac3c42b3..3697da742f08cbf022f90dc078292d67566a3156 100644
--- a/chrome/browser/resources/gaia_auth_host/gaia_auth_host.js
+++ b/chrome/browser/resources/gaia_auth_host/gaia_auth_host.js
@@ -342,6 +342,13 @@ cr.define('cr.login', function() {
 
       if (msg.method == 'loginUILoaded') {
         cr.dispatchSimpleEvent(this, 'ready');
+        // Declare __login_ui_initialized in window object so it could be read
+        // directly in telemetry tests.
+        if (msg.load_start_ts && !window.__login_ui_initialized) {
+          chrome.send('loginUIInitialized',
+                      [msg.load_start_ts, msg.load_stop_ts]);
+          window.__login_ui_initialized = true;
+        }
         return;
       }