ipc: Remove unnecessary attachment broker plumbing.

The original design was to pass around an instance of an attachment broker. The
new design uses a single global, and no longer needs any plumbing. This CL
removes the last vestiges of the plumbing.

BUG=493414

Review URL: https://codereview.chromium.org/1354973006

Cr-Commit-Position: refs/heads/master@{#350471}
diff --git a/ipc/ipc_fuzzing_tests.cc b/ipc/ipc_fuzzing_tests.cc
index 9cc92c4..769922d0 100644
--- a/ipc/ipc_fuzzing_tests.cc
+++ b/ipc/ipc_fuzzing_tests.cc
@@ -259,7 +259,7 @@
   base::MessageLoopForIO main_message_loop;
   FuzzerServerListener listener;
   scoped_ptr<IPC::Channel> channel(IPC::Channel::CreateClient(
-      IPCTestBase::GetChannelName("FuzzServerClient"), &listener, nullptr));
+      IPCTestBase::GetChannelName("FuzzServerClient"), &listener));
   CHECK(channel->Connect());
   listener.Init(channel.get());
   base::MessageLoop::current()->Run();