Use base namespace for string16 in components and cloud_print.

cloud_print/virtual_driver/win/virtual_driver_helpers.h

Index: cloud_print/virtual_driver/win/virtual_driver_helpers.h
diff --git a/cloud_print/virtual_driver/win/virtual_driver_helpers.h b/cloud_print/virtual_driver/win/virtual_driver_helpers.h
index 7a275ac9aeb9c31dcb08ceac5ec69262deec2791..426265fc2d75000c5615afbfb8ca302f4bbeb435 100644
--- a/cloud_print/virtual_driver/win/virtual_driver_helpers.h
+++ b/cloud_print/virtual_driver/win/virtual_driver_helpers.h
@@ -19,10 +19,11 @@ namespace cloud_print {
 bool IsSystem64Bit();
 
 // Convert an HRESULT to a localized string and display it in a message box.
-void DisplayWindowsMessage(HWND hwnd, HRESULT hr, const string16 &caption);
+void DisplayWindowsMessage(HWND hwnd, HRESULT hr,
+                           const base::string16 &caption);
 
 // Returns the correct port monitor DLL file name for the current machine.
-string16 GetPortMonitorDllName();
+base::string16 GetPortMonitorDllName();
 
 // Gets the standard install path for "version 3" print drivers.
 HRESULT GetPrinterDriverDir(base::FilePath* path);