Bug 1499726 - make main-thread XHR timer setting slightly more efficient; r=baku

Instead of creating a timer and then setting the timer's target, we can
determine the timer's target and pass it in directly when the timer is
created.  This reordering of steps is slightly more efficient, since
SetTarget() is both a virtual call and requires locking, both of which
can be skipped if we know the target at timer creation time.

dom/xhr/XMLHttpRequestMainThread.cpp

@@ -3142,13 +3142,13 @@ XMLHttpRequestMainThread::SetTimeout(uint32_t aTimeout, ErrorResult& aRv)
   }
 }
 
-void
-XMLHttpRequestMainThread::SetTimerEventTarget(nsITimer* aTimer)
+nsIEventTarget*
+XMLHttpRequestMainThread::GetTimerEventTarget()
 {
   if (nsCOMPtr<nsIGlobalObject> global = GetOwnerGlobal()) {
-    nsCOMPtr<nsIEventTarget> target = global->EventTargetFor(TaskCategory::Other);
-    aTimer->SetTarget(target);
+    return global->EventTargetFor(TaskCategory::Other);
   }
+  return nullptr;
 }
 
 nsresult
@@ -3183,8 +3183,7 @@ XMLHttpRequestMainThread::StartTimeoutTimer()
   }
 
   if (!mTimeoutTimer) {
-    mTimeoutTimer = NS_NewTimer();
-    SetTimerEventTarget(mTimeoutTimer);
+    mTimeoutTimer = NS_NewTimer(GetTimerEventTarget());
   }
   uint32_t elapsed =
     (uint32_t)((PR_Now() - mRequestSentTime) / PR_USEC_PER_MSEC);
@@ -3601,8 +3600,7 @@ void
 XMLHttpRequestMainThread::StartProgressEventTimer()
 {
   if (!mProgressNotifier) {
-    mProgressNotifier = NS_NewTimer();
-    SetTimerEventTarget(mProgressNotifier);
+    mProgressNotifier = NS_NewTimer(GetTimerEventTarget());
   }
   if (mProgressNotifier) {
     mProgressTimerIsActive = true;
@@ -3628,8 +3626,7 @@ XMLHttpRequestMainThread::MaybeStartSyncTimeoutTimer()
     return eErrorOrExpired;
   }
 
-  mSyncTimeoutTimer = NS_NewTimer();
-  SetTimerEventTarget(mSyncTimeoutTimer);
+  mSyncTimeoutTimer = NS_NewTimer(GetTimerEventTarget());
   if (!mSyncTimeoutTimer) {
     return eErrorOrExpired;
   }