Cast: make global_instance_ more strict

chrome/renderer/media/cast_ipc_dispatcher.cc

Index: chrome/renderer/media/cast_ipc_dispatcher.cc
diff --git a/chrome/renderer/media/cast_ipc_dispatcher.cc b/chrome/renderer/media/cast_ipc_dispatcher.cc
index 3d71e922035e48bc9cab52832ae859d8377b1060..feff41dd8704b055b78d00dea6357b5d4d06f49c 100644
--- a/chrome/renderer/media/cast_ipc_dispatcher.cc
+++ b/chrome/renderer/media/cast_ipc_dispatcher.cc
@@ -20,8 +20,7 @@ CastIPCDispatcher::CastIPCDispatcher(
 
 CastIPCDispatcher::~CastIPCDispatcher() {
   DCHECK(io_message_loop_->BelongsToCurrentThread());
-  // Unfortunately, you do not always get a OnFilterRemoved call.
-  global_instance_ = NULL;
+  DCHECK(!global_instance_);
 }
 
 CastIPCDispatcher* CastIPCDispatcher::Get() {
@@ -74,7 +73,6 @@ void CastIPCDispatcher::OnFilterRemoved() {
 void CastIPCDispatcher::OnChannelClosing() {
   DCHECK(io_message_loop_->BelongsToCurrentThread());
   DCHECK_EQ(this, global_instance_);
-  channel_ = NULL;
 }
 
 void CastIPCDispatcher::OnReceivedPacket(