ipc: Use a global for the process's attachment broker.

content/public/test/mock_render_thread.cc

Index: content/public/test/mock_render_thread.cc
diff --git a/content/public/test/mock_render_thread.cc b/content/public/test/mock_render_thread.cc
index dee8514b68c6108e252a3bf5b467c706dac67f61..2d6fd6edc4b849a630cb663788712277fd4a4d3b 100644
--- a/content/public/test/mock_render_thread.cc
+++ b/content/public/test/mock_render_thread.cc
@@ -189,10 +189,6 @@ void MockRenderThread::ReleaseCachedFonts() {
 
 #endif  // OS_WIN
 
-IPC::AttachmentBroker* MockRenderThread::GetAttachmentBroker() {
-  return nullptr;
-}
-
 ServiceRegistry* MockRenderThread::GetServiceRegistry() {
   return NULL;
 }