Implement browser side of PPB_Printing resource.

ppapi/proxy/plugin_dispatcher.cc

Index: ppapi/proxy/plugin_dispatcher.cc
diff --git a/ppapi/proxy/plugin_dispatcher.cc b/ppapi/proxy/plugin_dispatcher.cc
index d48dbfb5de32949041a0b93583e94379f0d68ed3..8dbca8d23d38d4a8aff6d8c7bce345d7cfa66746 100644
--- a/ppapi/proxy/plugin_dispatcher.cc
+++ b/ppapi/proxy/plugin_dispatcher.cc
@@ -102,6 +102,10 @@ PluginDispatcher* PluginDispatcher::GetForResource(const Resource* resource) {
 
 // static
 const void* PluginDispatcher::GetBrowserInterface(const char* interface_name) {
+  DCHECK(interface_name) << "|interface_name| is null. Did you forget to add "
+      "the |interface_name()| template function to the interface's C++ "
+      "wrapper?";
+
   return InterfaceList::GetInstance()->GetInterfaceForPPB(interface_name);
 }