Implement browser side of PPB_Printing resource.

ppapi/thunk/enter.cc

Index: ppapi/thunk/enter.cc
diff --git a/ppapi/thunk/enter.cc b/ppapi/thunk/enter.cc
index d5d822b5ae76ce3f7464d82a0efbebf6c4c88e95..c89c9695b5a09b4486510d0e9be5430aeea6baaa 100644
--- a/ppapi/thunk/enter.cc
+++ b/ppapi/thunk/enter.cc
@@ -62,7 +62,8 @@ EnterBase::~EnterBase() {
   // callback_ is cleared any time it is run, scheduled to be run, or once we
   // know it will be completed asynchronously. So by this point it should be
   // NULL.
-  DCHECK(!callback_);
+  DCHECK(!callback_) << "|callback_| is not NULL. Did you forget to call "
+      "|EnterBase::SetResult| in the interface's thunk?";
 }
 
 int32_t EnterBase::SetResult(int32_t result) {