PPAPI: Make PPB_MessageLoop_Dev::GetForMainThread work

ppapi/shared_impl/resource.h

 // 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.
 
 #ifndef PPAPI_SHARED_IMPL_RESOURCE_H_
 #define PPAPI_SHARED_IMPL_RESOURCE_H_
 
 #include <stddef.h>  // For NULL.
 
 #include <string>
@@ skipping 89 matching lines @@
   // For constructing "impl" (non-proxied) objects, this just takes the
   // associated instance, and generates a new resource ID. The host resource
   // will be the same as the newly-generated resource ID. For all objects in
   // the renderer (host) process, you'll use this constructor and call it with
   // OBJECT_IS_IMPL.
   //
   // For proxied objects, this will create an "instance-only" object which
   // lives only in the plugin and doesn't have a corresponding object in the
   // host. If you have a host resource ID, use the constructor below which
   // takes that HostResource value.
-  explicit Resource(ResourceObjectType type, PP_Instance instance);
+  Resource(ResourceObjectType type, PP_Instance instance);
 
   // For constructing given a host resource.
   //
   // For OBJECT_IS_PROXY objects, this takes the resource generated in the host
   // side, stores it, and allocates a "local" resource ID for use in the
   // current process.
   //
   // For OBJECT_IS_IMPL, the host resource ID must be 0, since there should be
   // no host resource generated (impl objects should generate their own). The
   // reason for supporting this constructor at all for the IMPL case is that
   // some shared objects use a host resource for both modes to keep things the
   // same.
-  explicit Resource(ResourceObjectType type,
-                    const HostResource& host_resource);
+  Resource(ResourceObjectType type, const HostResource& host_resource);
+
+  // Constructor for untracked objects. These have no associated instance. Use
+  // this with care, as the object is likely to persist for the lifetime of the
+  // plugin module. This is appropriate in some rare cases, like the
+  // PPB_MessageLoop resource for the main thread.
+  struct Untracked {};
+  explicit Resource(Untracked);
 
   virtual ~Resource();
 
   PP_Instance pp_instance() const { return host_resource_.instance(); }
 
   // Returns the resource ID for this object in the current process without
   // adjusting the refcount. See also GetReference().
   PP_Resource pp_resource() const { return pp_resource_; }
 
   // Returns the host resource which identifies the resource in the host side
@@ skipping 66 matching lines @@
 #define DEFINE_RESOURCE_CAST(RESOURCE) \
   template<> inline thunk::RESOURCE* Resource::GetAs() { \
     return As##RESOURCE(); \
   }
 FOR_ALL_PPAPI_RESOURCE_APIS(DEFINE_RESOURCE_CAST)
 #undef DEFINE_RESOURCE_CAST
 
 }  // namespace ppapi
 
 #endif  // PPAPI_SHARED_IMPL_RESOURCE_H_