1325254 - optimize TimerThread data structures

1325254 P1 Make TimerThread::mTimers store RefPtr<nsTimerImpl> objects.

1325254 P2 Make TimerThread list store an entry struct and just drop nsTimerImpl ref on cancel.

1325254 P3 Sort TimerThread list as a binary heap.

1325254 P4 Dynamically allocate Entry structs stored in TimerThread::mTimers.

1325254 P5 Make nsITimer::Cancel() O(c).

xpcom/threads/TimerThread.h

@@ -19,6 +19,8 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/Monitor.h"
 
+#include <algorithm>
+
 namespace mozilla {
 class TimeStamp;
 } // namespace mozilla
@@ -60,12 +62,12 @@ private:
   mozilla::Atomic<bool> mInitInProgress;
   bool    mInitialized;
 
-  // These two internal helper methods must be called while mMonitor is held.
-  // AddTimerInternal returns the position where the timer was added in the
-  // list, or -1 if it failed.
-  int32_t AddTimerInternal(nsTimerImpl* aTimer);
+  // These internal helper methods must be called while mMonitor is held.
+  // AddTimerInternal returns false if the insertion failed.
+  bool    AddTimerInternal(nsTimerImpl* aTimer);
   bool    RemoveTimerInternal(nsTimerImpl* aTimer);
-  void    ReleaseTimerInternal(nsTimerImpl* aTimer);
+  void    RemoveLeadingCanceledTimersInternal();
+  void    RemoveFirstTimerInternal();
 
   already_AddRefed<nsTimerImpl> PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef);
 
@@ -81,7 +83,43 @@ private:
   bool mNotified;
   bool mSleeping;
 
-  nsTArray<nsTimerImpl*> mTimers;
+  class Entry final : public nsTimerImplHolder
+  {
+    const TimeStamp mTimeout;
+
+  public:
+    Entry(const TimeStamp& aMinTimeout, const TimeStamp& aTimeout,
+          nsTimerImpl* aTimerImpl)
+      : nsTimerImplHolder(aTimerImpl)
+      , mTimeout(std::max(aMinTimeout, aTimeout))
+    {
+    }
+
+    nsTimerImpl*
+    Value() const
+    {
+      return mTimerImpl;
+    }
+
+    already_AddRefed<nsTimerImpl>
+    Take()
+    {
+      if (mTimerImpl) {
+        mTimerImpl->SetHolder(nullptr);
+      }
+      return mTimerImpl.forget();
+    }
+
+    static bool
+    UniquePtrLessThan(UniquePtr<Entry>& aLeft, UniquePtr<Entry>& aRight)
+    {
+      // This is reversed because std::push_heap() sorts the "largest" to
+      // the front of the heap.  We want that to be the earliest timer.
+      return aRight->mTimeout < aLeft->mTimeout;
+    }
+  };
+
+  nsTArray<UniquePtr<Entry>> mTimers;
 };
 
 struct TimerAdditionComparator