Cleanup histogram syncronizer a bit...

chrome/browser/metrics/histogram_synchronizer.h

Index: chrome/browser/metrics/histogram_synchronizer.h
===================================================================
--- chrome/browser/metrics/histogram_synchronizer.h	(revision 64486)
+++ chrome/browser/metrics/histogram_synchronizer.h	(working copy)
@@ -60,7 +60,7 @@
   // signal the waiting process or call the callback function. Returns true when
   // we receive histograms from the last of N renderers that were contacted for
   // an update. This is called on IO Thread.
-  bool RecordRendererHistogram(int sequence_number);
+  bool DecrementPendingRenderers(int sequence_number);
 
   void SetCallbackTaskToCallAfterGettingHistograms(
       MessageLoop* callback_thread, Task* callback_task);
@@ -84,8 +84,8 @@
   // singular IO thread and we don't need to worry about locks.
   bool IsOnIoThread();
 
-  // This lock_ protects access to sequence number and
-  // synchronous_renderers_pending_.
+  // This lock_ protects access to next_sequence_number_,
+  // synchronous_renderers_pending_, and synchronous_sequence_number_.
   Lock lock_;
 
   // This condition variable is used to block caller of the synchronous request
@@ -110,27 +110,27 @@
   // sure a response from a renderer is associated with the current round of
   // requests (and not merely a VERY belated prior response).
   // next_available_sequence_number_ is the next available number (used to
-  // avoid reuse for a long time).
+  // avoid reuse for a long time).  Access is protected by lock_.
   int next_available_sequence_number_;
 
   // The sequence number used by the most recent asynchronous update request to
-  // contact all renderers.
+  // contact all renderers.  Access is only permitted on the IO thread.
   int async_sequence_number_;
 
   // The number of renderers that have not yet responded to requests (as part of
-  // an asynchronous update).
+  // an asynchronous update).  Access is only permitted on the IO thread.
   int async_renderers_pending_;
 
   // The time when we were told to start the fetch histograms asynchronously
-  // from renderers.
+  // from renderers.  Access is only permitted on the IO thread.
   base::TimeTicks async_callback_start_time_;
 
   // The sequence number used by the most recent synchronous update request to
-  // contact all renderers.
+  // contact all renderers.  Protected by lock_.
   int synchronous_sequence_number_;
 
   // The number of renderers that have not yet responded to requests (as part of
-  // a synchronous update).
+  // a synchronous update).  Protected by lock_.
   int synchronous_renderers_pending_;
 
   // This singleton instance should be started during the single threaded