Don't attempt to forward declare StringPiece.

ui/base/resource/data_pack.h

Index: ui/base/resource/data_pack.h
diff --git a/ui/base/resource/data_pack.h b/ui/base/resource/data_pack.h
index 0cc3b31514aeb53f0d9bc1ee2082d9d6db5ac746..a8bca0e0e40e0a62c63077e20236b2efc75cd500 100644
--- a/ui/base/resource/data_pack.h
+++ b/ui/base/resource/data_pack.h
@@ -14,15 +14,12 @@
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/string_piece.h"
 #include "ui/base/ui_export.h"
 
 class FilePath;
 class RefCountedStaticMemory;
 
-namespace base {
-class StringPiece;
-}
-
 namespace file_util {
 class MemoryMappedFile;
 }