Implement browser side of PPB_Printing resource.

ppapi/thunk/enter.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ppapi/thunk/enter.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
 #include "base/stringprintf.h"
@@ skipping 44 matching lines @@
       retval_(PP_OK) {
   callback_ = new TrackedCallback(resource_, callback);
 
   // TODO(dmichael) validate that threads have an associated message loop.
 }
 
 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) {
   if (!callback_) {
     // It doesn't make sense to call SetResult if there is no callback.
     NOTREACHED();
     retval_ = result;
     return result;
   }
   if (result == PP_OK_COMPLETIONPENDING) {
@@ skipping 188 matching lines @@
     : EnterBase(),
       functions_(PpapiGlobals::Get()->GetResourceCreationAPI(instance)) {
   SetStateForFunctionError(instance, functions_, true);
 }
 
 EnterResourceCreationNoLock::~EnterResourceCreationNoLock() {
 }
 
 }  // namespace thunk
 }  // namespace ppapi