Add protocol APIs to CommandDispatcher.

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

 // 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.
 
 #ifndef IOS_SHARED_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_
 #define IOS_SHARED_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_
 
 #import <Foundation/Foundation.h>
 
 // CommandDispatcher allows coordinators to register as command handlers for
 // specific selectors.  Other objects can call these methods on the dispatcher,
 // which in turn will forward the call to the registered handler.
 @interface CommandDispatcher : NSObject
 
 // Registers the given |target| to receive forwarded messages for the given
 // |selector|.
 - (void)startDispatchingToTarget:(id)target forSelector:(SEL)selector;
 
 // Removes forwarding registration for the given |selector|.
 - (void)stopDispatchingForSelector:(SEL)selector;
 
+// Registers the given |target| to receive forwarded messages for the methods of
+// the given |protocol|. Only required instance methods are registered. The
+// other definitions in the protocol are ignored.
+- (void)startDispatchingToTarget:(id)target forProtocol:(Protocol*)protocol;
+
+// Removes forwarding registration for the given |selector|. Only dispatching to
+// required instance methods is removed. The other definitions in the protocol
+// are ignored.
+- (void)stopDispatchingForProtocol:(Protocol*)protocol;
+
 // Removes all forwarding registrations for the given |target|.
 - (void)stopDispatchingToTarget:(id)target;
 
 @end
 
 #endif  // IOS_SHARED_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_