Reland 95309. Add a template to handle properly issuing completion callbacks.

ppapi/proxy/ppb_url_loader_proxy.cc

Index: ppapi/proxy/ppb_url_loader_proxy.cc
===================================================================
--- ppapi/proxy/ppb_url_loader_proxy.cc	(revision 95737)
+++ ppapi/proxy/ppb_url_loader_proxy.cc	(working copy)
@@ -517,22 +517,13 @@
   // TODO(brettw) have a way to check for out-of-memory.
   info->read_buffer.resize(bytes_to_read);
 
-  CompletionCallback callback = callback_factory_.NewOptionalCallback(
-      &PPB_URLLoader_Proxy::OnReadCallback, info);
-
-  EnterHostFromHostResource<PPB_URLLoader_API> enter(loader);
-  int32_t result = PP_ERROR_BADRESOURCE;
+  EnterHostFromHostResourceForceCallback<PPB_URLLoader_API> enter(
+      loader, callback_factory_, &PPB_URLLoader_Proxy::OnReadCallback, info);
   if (enter.succeeded()) {
-    result = enter.object()->ReadResponseBody(
+    enter.SetResult(enter.object()->ReadResponseBody(
         const_cast<char*>(info->read_buffer.c_str()),
-        bytes_to_read, callback.pp_completion_callback());
+        bytes_to_read, enter.callback()));
   }
-  if (result != PP_OK_COMPLETIONPENDING) {
-    // Send error (or perhaps success for synchronous reads) back to plugin.
-    // The callback function is already set up to do this and also delete the
-    // callback info.
-    callback.Run(result);
-  }
 }
 
 void PPB_URLLoader_Proxy::OnMsgFinishStreamingToFile(