[ios clean] Move dispatcher out of Browser.

ios/shared/chrome/browser/ui/commands/command_dispatcher.mm

Index: ios/shared/chrome/browser/ui/commands/command_dispatcher.mm
diff --git a/ios/shared/chrome/browser/ui/commands/command_dispatcher.mm b/ios/shared/chrome/browser/ui/commands/command_dispatcher.mm
index 461edefacadc1695f1127445b5276d903bdef30c..54dbe36e4502a15cb3aca3d8841c7c9869203283 100644
--- a/ios/shared/chrome/browser/ui/commands/command_dispatcher.mm
+++ b/ios/shared/chrome/browser/ui/commands/command_dispatcher.mm
@@ -21,7 +21,9 @@
 }
 
 - (void)startDispatchingToTarget:(id)target forSelector:(SEL)selector {
-  DCHECK(_forwardingTargets.find(selector) == _forwardingTargets.end());
+  DCHECK(_forwardingTargets.find(selector) == _forwardingTargets.end())
+      << "Already dispatching to a target for "
+      << base::SysNSStringToUTF8(NSStringFromSelector(selector));
 
   _forwardingTargets[selector] = target;
 }