PPAPI: Make PPB_MessageLoop_Dev::GetForMainThread work

ppapi/shared_impl/resource.h

Index: ppapi/shared_impl/resource.h
diff --git a/ppapi/shared_impl/resource.h b/ppapi/shared_impl/resource.h
index 05dc78361cb1294fe5bbd63db03b9c90bf144a7e..01cd61b9f2a51b23c646907407dc72de45cb031d 100644
--- a/ppapi/shared_impl/resource.h
+++ b/ppapi/shared_impl/resource.h
@@ -107,7 +107,7 @@ class PPAPI_SHARED_EXPORT Resource : public base::RefCounted<Resource> {
   // 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.
   //
@@ -120,8 +120,14 @@ class PPAPI_SHARED_EXPORT Resource : public base::RefCounted<Resource> {
   // 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();