Add ContextMenuCoordinator.

ios/chrome/browser/ui/context_menu/context_menu_coordinator.h

+// Copyright 2016 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_CHROME_BROWSER_UI_CONTEXT_MENU_CONTEXT_MENU_COORDINATOR_H_
+#define IOS_CHROME_BROWSER_UI_CONTEXT_MENU_CONTEXT_MENU_COORDINATOR_H_
+
+#import <UIKit/UIKit.h>
+
+#include "base/ios/block_types.h"
+
+namespace web {
+struct ContextMenuParams;
+}
+
+// Abstracts displaying context menus for all device form factors. Will show a
+// sheet on the phone and use a popover on a tablet.
+@interface ContextMenuCoordinator : NSObject
+
+// Whether the context menu is visible.
+@property(nonatomic, readonly, getter=isVisible) BOOL visible;
+
+// Initializes with details provided in |params|. Context menu will be presented
+// from |viewController|.
+- (instancetype)initWithViewController:(UIViewController*)viewController
+                                params:(const web::ContextMenuParams&)params;
+
+// Adds an item at the end of the menu.

[marq (ping after 24h), 2016/05/21 09:39:58]

What happens if this is called after -start is called? After -stop is called?

[michaeldo, 2016/05/23 21:19:07]

I've updated the comment and code to early return if |addItemWith...| is called
after start.

+- (void)addItemWithTitle:(NSString*)title action:(ProceduralBlock)action;
+
+// Displays the context menu.
+- (void)start;
+// Dismisses displayed context menu.
+- (void)stop;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_UI_CONTEXT_MENU_CONTEXT_MENU_COORDINATOR_H_