Bug 790213 - Add telemetry probe for page load time. r=dao

browser/base/content/browser.js

@@ -4472,6 +4472,15 @@ var TabsProgressListener = {
     }
 #endif
 
+    // Collect telemetry data about tab load times.
+    if (aWebProgress.DOMWindow == aWebProgress.DOMWindow.top &&
+        aStateFlags & Ci.nsIWebProgressListener.STATE_IS_WINDOW) {
+      if (aStateFlags & Ci.nsIWebProgressListener.STATE_START)
+        TelemetryStopwatch.start("FX_PAGE_LOAD_MS", aBrowser);
+      else if (aStateFlags & Ci.nsIWebProgressListener.STATE_STOP)
+        TelemetryStopwatch.finish("FX_PAGE_LOAD_MS", aBrowser);
+    }
+
     // Attach a listener to watch for "click" events bubbling up from error
     // pages and other similar page. This lets us fix bugs like 401575 which
     // require error page UI to do privileged things, without letting error

toolkit/components/telemetry/Histograms.json

@@ -1956,6 +1956,12 @@
     "n_buckets": 20,
     "description": "Firefox: Time taken to open a new browser window (ms)"
   },
+  "FX_PAGE_LOAD_MS": {
+    "kind": "exponential",
+    "high": "10000",
+    "n_buckets": 20,
+    "description": "Firefox: Time taken to load a page (ms)"
+  },
   "FX_THUMBNAILS_CAPTURE_TIME_MS": {
     "kind": "exponential",
     "high": "500",