add default constructor to PendingTask; compilation fix for VS2012

base/pending_task.h

Index: base/pending_task.h
diff --git a/base/pending_task.h b/base/pending_task.h
index 78491ee1a82b2ca778bbc91de3c7ae39d033a332..945771b000d0c6c0cb5add5a680ca494719eb7dc 100644
--- a/base/pending_task.h
+++ b/base/pending_task.h
@@ -18,6 +18,7 @@ namespace base {
 // Contains data about a pending task. Stored in TaskQueue and DelayedTaskQueue
 // for use by classes that queue and execute tasks.
 struct BASE_EXPORT PendingTask : public TrackingInfo {
+  PendingTask();
   PendingTask(const tracked_objects::Location& posted_from,
               const Closure& task);
   PendingTask(const tracked_objects::Location& posted_from,