Bug 1376038 - Part 1: Use a cached ghost window value for the distinguished amount. r=mccr8

We already periodically calculate the ghost window amount after cycle
collection, this just uses a cached value of that for the distinguished amount.
This avoids the overhead of a recalculating the value when reporting telemetry.

dom/base/nsWindowMemoryReporter.cpp

@@ -34,7 +34,8 @@ const int32_t kTimeBetweenChecks = 45; /* seconds */
 nsWindowMemoryReporter::nsWindowMemoryReporter()
   : mLastCheckForGhostWindows(TimeStamp::NowLoRes()),
     mCycleCollectorIsRunning(false),
-    mCheckTimerWaitingForCCEnd(false)
+    mCheckTimerWaitingForCCEnd(false),
+    mGhostWindowCount(0)
 {
 }
 
@@ -757,6 +758,7 @@ nsWindowMemoryReporter::CheckForGhostWindows(
   // if it's not null.
   uint32_t ghostTimeout = GetGhostTimeout();
   TimeStamp now = mLastCheckForGhostWindows;
+  mGhostWindowCount = 0;
   for (auto iter = mDetachedWindows.Iter(); !iter.Done(); iter.Next()) {
     nsWeakPtr weakKey = do_QueryInterface(iter.Key());
     nsCOMPtr<mozIDOMWindow> iwindow = do_QueryReferent(weakKey);
@@ -809,6 +811,7 @@ nsWindowMemoryReporter::CheckForGhostWindows(
         // that is not null.
         if (aOutGhostIDs && window) {
           aOutGhostIDs->PutEntry(window->WindowID());
+          mGhostWindowCount++;
         }
       }
     }
@@ -821,9 +824,7 @@ NS_IMPL_ISUPPORTS(nsWindowMemoryReporter::GhostWindowsReporter,
 /* static */ int64_t
 nsWindowMemoryReporter::GhostWindowsReporter::DistinguishedAmount()
 {
-  nsTHashtable<nsUint64HashKey> ghostWindows;
-  sWindowReporter->CheckForGhostWindows(&ghostWindows);
-  return ghostWindows.Count();
+  return sWindowReporter->mGhostWindowCount;
 }
 
 void