Bug 715947 - add telemetry for timers fired; r=bz

dom/base/nsGlobalWindow.cpp

@@ -9181,6 +9181,8 @@ nsGlobalWindow::RunTimeout(nsTimeout *aTimeout)
   // the logic in ResetTimersForNonBackgroundWindow will need to change.
   mTimeoutInsertionPoint = &dummy_timeout;
 
+  Telemetry::AutoCounter<Telemetry::DOM_TIMERS_FIRED_PER_NATIVE_TIMEOUT> timeoutsRan;
+
   for (timeout = FirstTimeout();
        timeout != &dummy_timeout && !IsFrozen();
        timeout = nextTimeout) {
@@ -9232,6 +9234,7 @@ nsGlobalWindow::RunTimeout(nsTimeout *aTimeout)
     nsTimeout *last_running_timeout = mRunningTimeout;
     mRunningTimeout = timeout;
     timeout->mRunning = true;
+    ++timeoutsRan;
 
     // Push this timeout's popup control state, which should only be
     // eabled the first time a timeout fires that was created while

toolkit/components/telemetry/TelemetryHistograms.h

@@ -319,6 +319,11 @@ HISTOGRAM(HTML_REFLOW_MS, 1, 3000, 10, EXPONENTIAL, "HTML reflows (ms)")
 HISTOGRAM(XUL_INITIAL_FRAME_CONSTRUCTION, 1, 3000, 10, EXPONENTIAL, "initial xul frame construction")
 HISTOGRAM_BOOLEAN(XMLHTTPREQUEST_ASYNC_OR_SYNC, "Type of XMLHttpRequest, async or sync")
 
+/**
+ * DOM telemetry.
+ */
+HISTOGRAM(DOM_TIMERS_FIRED_PER_NATIVE_TIMEOUT, 1, 3000, 10, EXPONENTIAL, "DOM: Timer handlers called per native timer expiration")
+
 /**
  * DOM Storage telemetry.
  */