Implement idle task scheduling.

Source/wtf/DoubleBufferedDeque.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DoubleBufferedDeque_h
 #define DoubleBufferedDeque_h
 
 #include "wtf/Deque.h"
 #include "wtf/Noncopyable.h"
 
 namespace WTF {
 
 // A helper class for managing double buffered deques, typically where the client locks when appending or swapping.
 template <typename T> class DoubleBufferedDeque {
     WTF_MAKE_NONCOPYABLE(DoubleBufferedDeque);
 public:
+    typedef typename PassTraits<T>::PassType PassType;
     DoubleBufferedDeque()
         : m_activeIndex(0) { }
 
-    void append(const T& value)
+    void append(const PassType& value)

[Sami, 2014/09/24 14:00:40]

Is this an optimization to avoid copies or something else?

[rmcilroy, 2014/09/29 17:42:58]

This is required to be able to create a DoubleBufferedDeque with OwnPtr or
RefPtr because the append function needs to use the pass type (PassOwnPtr)
rather than an OwnPtr (which wouldn't compile).  It shouldn't make any
difference on templated types which are not OwnPtr or RefPtr (PassType == T in
those cases).

     {
         m_queue[m_activeIndex].append(value);
     }
 

[picksi1, 2014/09/25 08:53:31]

FWIW during my refactor I stopped using the DoubleBufferedDeque as hand-rolling
my own double buffered deque was easy and allowed me to add more functionality
to it without bloating this class. We should keep an eye on this if we find
ourselves making more changes to this class.

[rmcilroy, 2014/09/29 17:42:58]

I would much prefer to fix this DoubleBufferedDeque such that it is useful
rather than hand rolling our own if at all possible - is there any reason you
couldn't add the functionality you needed here  in your refactor?

[picksi1, 2014/09/30 09:13:51]

My changes felt against the spirit of the DoubleBufferedDeque - I was taking
tasks off the 'hidden' deque. I also (incorrectly) thought this was a
fundamental low-level class that would cause turmoil in the Blink masses if I'd
changed it ... turns out that Alex wrote it a couple of weeks ago, so I should
have been less scared. :)

[rmcilroy, 2014/09/30 09:56:19]

Makes sense. You should never be scared of causing turmoil for the Blink masses
though ;). 

     bool isEmpty() const
     {
         return m_queue[m_activeIndex].isEmpty();
     }
 
     Deque<T>& swapBuffers()
     {
         int oldIndex = m_activeIndex;
         m_activeIndex ^= 1;
         ASSERT(m_queue[m_activeIndex].isEmpty());
         return m_queue[oldIndex];
     }
 
 private:
     Deque<T> m_queue[2];
     int m_activeIndex;
 };
 
 } // namespace WTF
 
 using WTF::DoubleBufferedDeque;
 
 #endif // DoubleBufferedDeque_h