Don't attempt to forward declare StringPiece.

base/sys_string_conversions.h

Index: base/sys_string_conversions.h
diff --git a/base/sys_string_conversions.h b/base/sys_string_conversions.h
index d2f4d1b5be6b3d142c892d70f87eb8d327ff27ba..2be248de864754bf2762a8809734397492b3558a 100644
--- a/base/sys_string_conversions.h
+++ b/base/sys_string_conversions.h
@@ -15,6 +15,7 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/string16.h"
+#include "base/string_piece.h"
 
 #if defined(OS_MACOSX)
 #include <CoreFoundation/CoreFoundation.h>
@@ -27,8 +28,6 @@ class NSString;
 
 namespace base {
 
-class StringPiece;
-
 // Converts between wide and UTF-8 representations of a string. On error, the
 // result is system-dependent.
 BASE_EXPORT std::string SysWideToUTF8(const std::wstring& wide);