[ios clean] Move dispatcher out of Browser.

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

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/shared/chrome/browser/ui/commands/command_dispatcher.h"
 
 #include <objc/runtime.h>
 #include <unordered_map>
 #include <vector>
 
 #include "base/logging.h"
 #include "base/strings/sys_string_conversions.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 @implementation CommandDispatcher {
   // Stores which target to forward to for a given selector.
   std::unordered_map<SEL, __weak id> _forwardingTargets;
 }
 
 - (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;
 }
 
 - (void)startDispatchingToTarget:(id)target forProtocol:(Protocol*)protocol {
   unsigned int methodCount;
   objc_method_description* requiredInstanceMethods =
       protocol_copyMethodDescriptionList(protocol, YES /* isRequiredMethod */,
                                          YES /* isInstanceMethod */,
                                          &methodCount);
@@ skipping 39 matching lines @@
 // Overridden to forward messages to registered handlers.
 - (id)forwardingTargetForSelector:(SEL)selector {
   auto target = _forwardingTargets.find(selector);
   if (target != _forwardingTargets.end()) {
     return target->second;
   }
   return [super forwardingTargetForSelector:selector];
 }
 
 @end