Add function to CancelableTaskTracker and convert BootTimeLoader (take 2)

chrome/common/cancelable_task_tracker.h

Index: chrome/common/cancelable_task_tracker.h
diff --git a/chrome/common/cancelable_task_tracker.h b/chrome/common/cancelable_task_tracker.h
index 0a7289b03d5e25729785b11f70605ce4c177cee8..86450c30c486648bf9cd2d72a618521bf6c4c3f2 100644
--- a/chrome/common/cancelable_task_tracker.h
+++ b/chrome/common/cancelable_task_tracker.h
@@ -28,6 +28,8 @@
 // commonly used to cancel all outstanding tasks.
 //
 // 2. Both task and reply are deleted on the originating thread.
+//
+// 3. IsCanceledCallback is thread safe and can be run or deleted on any thread.
 
 #ifndef CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
 #define CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
@@ -53,6 +55,8 @@ class CancelableTaskTracker {
   typedef int64 TaskId;
   static const TaskId kBadTaskId;
 
+  typedef base::Callback<bool()> IsCanceledCallback;
+
   CancelableTaskTracker();
 
   // Cancels all tracked tasks.
@@ -67,6 +71,14 @@ class CancelableTaskTracker {
                           const base::Closure& task,
                           const base::Closure& reply);
 
+  // Creates a tracked TaskId and an associated IsCanceledCallback. Client can
+  // later call TryCancel() with the returned TaskId, and run |is_canceled_cb|
+  // to check whether the TaskId is canceled.
+  //
+  // Note. This function is used to address some special cancelation requirement
+  // in existing code. You SHOULD NOT need this function in new code.
+  TaskId NewTrackedTaskId(IsCanceledCallback* is_canceled_cb);
+
   // After calling this function, |task| and |reply| will not run. If the
   // cancelation happens when |task| is running or has finished running, |reply|
   // will not run. If |reply| is running or has finished running, cancellation