Fix occasional crash in remote commands code

The RemoteCommandsService depends on a reference to CloudPolicyClient, but
it doesn't get deleted before CloudPolicyClient is gone. This leads to dangling
pointer and will probably crash the browser.

This CL also remove code attempts to workaround this issue in crrev.com/330082

BUG=480982

Review URL: https://codereview.chromium.org/1134133005

Cr-Commit-Position: refs/heads/master@{#330497}

components/policy/core/common/cloud/cloud_policy_core.cc

@@ -49,6 +49,7 @@ void CloudPolicyCore::Disconnect() {
     FOR_EACH_OBSERVER(Observer, observers_, OnCoreDisconnecting(this));
   refresh_delay_.reset();
   refresh_scheduler_.reset();
+  remote_commands_service_.reset();
   service_.reset();
   client_.reset();
 }

components/policy/core/common/remote_commands/remote_commands_service.cc

@@ -22,6 +22,7 @@ RemoteCommandsService::RemoteCommandsService(
     scoped_ptr<RemoteCommandsFactory> factory,
     CloudPolicyClient* client)
     : factory_(factory.Pass()), client_(client), weak_factory_(this) {
+  DCHECK(client_);
   queue_.AddObserver(this);
 }
 
@@ -30,7 +31,7 @@ RemoteCommandsService::~RemoteCommandsService() {
 }
 
 bool RemoteCommandsService::FetchRemoteCommands() {
-  if (!client_ || !client_->is_registered())
+  if (!client_->is_registered())
     return false;
 
   if (command_fetch_in_progress_) {