Temp experiment to confirm theory in crbug.com/5555....

base/message_loop.h

 // Copyright (c) 2006-2008 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 BASE_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_H_
 
 #include <deque>
 #include <queue>
 #include <string>
@@ skipping 251 matching lines @@
     bool nestable;                // True if OK to dispatch from a nested loop.
 
     PendingTask(Task* task, bool nestable)
         : task(task), sequence_num(0), nestable(nestable) {
     }
 
     // Used to support sorting.
     bool operator<(const PendingTask& other) const;
   };
 
+  // TEMP HACK: This is the same as std::less, except it has a data member.
+  // This should cause its storage to be initialized, therefore avoiding a
+  // UMR in pop(). Experiment for crbug.com/5555.
+  template<class T>
+  struct LessComparatorHack {
+    LessComparatorHack() : junk(42) {}
+    bool operator()(const T& a, const T& b) const { return a < b; }
+    int junk;
+  };
+
   typedef std::queue<PendingTask> TaskQueue;
-  typedef std::priority_queue<PendingTask> DelayedTaskQueue;
+  typedef std::priority_queue<PendingTask, std::vector<PendingTask>,
+      LessComparatorHack<PendingTask> > DelayedTaskQueue;
 
 #if defined(OS_WIN)
   base::MessagePumpWin* pump_win() {
     return static_cast<base::MessagePumpWin*>(pump_.get());
   }
 #elif defined(OS_POSIX)
   base::MessagePumpLibevent* pump_libevent() {
     return static_cast<base::MessagePumpLibevent*>(pump_.get());
   }
 #endif
@@ skipping 200 matching lines @@
 #endif  // defined(OS_POSIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 #endif  // BASE_MESSAGE_LOOP_H_