Add comments to SimpleSynchronousEntry callbacks.

net/disk_cache/simple/simple_synchronous_entry.h

Index: net/disk_cache/simple/simple_synchronous_entry.h
diff --git a/net/disk_cache/simple/simple_synchronous_entry.h b/net/disk_cache/simple/simple_synchronous_entry.h
index c47ba018b03c5b0b3bc9e1f4d1a3131fa4d81ae8..082caeb050034fe23d235029c0fd069222fb4056 100644
--- a/net/disk_cache/simple/simple_synchronous_entry.h
+++ b/net/disk_cache/simple/simple_synchronous_entry.h
@@ -31,11 +31,16 @@ namespace disk_cache {
 // a single thread between synchronization points.
 class SimpleSynchronousEntry {
  public:
-  typedef base::Callback<void(SimpleSynchronousEntry*)>
+  // Callback type for the completion of creation of SimpleSynchronousEntry
+  // objects, for instance after OpenEntry() or CreateEntry(). |new_entry| is
+  // the newly created SimpleSynchronousEntry, constructed on the worker pool
+  // thread. |new_entry| == NULL in the case of error.
+  typedef base::Callback<void(SimpleSynchronousEntry* /* new_entry */)>
       SynchronousCreationCallback;
 
-  typedef base::Callback<void(int)>
-      SynchronousOperationCallback;
+  // Callback type for IO operations on an entry not requiring special callback
+  // arguments (e.g. Write). |result| is a net::Error result code.
+  typedef base::Callback<void(int /* result */)> SynchronousOperationCallback;
 
   static void OpenEntry(
       const base::FilePath& path,