Make FileStream::Seek async and add FileStream::SeekSync for sync operation

net/base/completion_callback.h

Index: net/base/completion_callback.h
diff --git a/net/base/completion_callback.h b/net/base/completion_callback.h
index 1d75bce1cccecc7f581e675a9dc997632e60e062..a519491549fcaf5a47c0d7042902c84ced4b7846 100644
--- a/net/base/completion_callback.h
+++ b/net/base/completion_callback.h
@@ -15,6 +15,10 @@ namespace net {
 // used to report a byte count or network error code.
 typedef base::Callback<void(int)> CompletionCallback;
 
+// 64bit version of callback specialization that takes a single int64 parameter.
+// Usually this is used to report a file offset, size or network error code.
+typedef base::Callback<void(int64)> Int64CompletionCallback;
+
 typedef base::CancelableCallback<void(int)> CancelableCompletionCallback;
 
 }  // namespace net