Expose thumbnails of pages to iOS share extensions.

ios/chrome/browser/ui/activity_services/chrome_activity_item_source.h

 // Copyright 2014 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_ACTIVITY_SERVICES_CHROME_ACTIVITY_ITEM_SOURCE_H_
 #define IOS_CHROME_BROWSER_UI_ACTIVITY_SERVICES_CHROME_ACTIVITY_ITEM_SOURCE_H_
 
 #import <Foundation/Foundation.h>
 #import <UIKit/UIKit.h>
 
-// Provides an URL and a subject to UIActivities. The subject is used by the
-// Mail UIActivities to populate the subject field.
-@interface UIActivityURLSource : NSObject<UIActivityItemSource>
-
-// Default initializer. |subject| and |url| must not be nil.
-- (instancetype)initWithURL:(NSURL*)url subject:(NSString*)subject;
-
-@end
+#import "ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.h"
 
 // Returns a text to the UIActivities that can take advantage of it.
 @interface UIActivityTextSource : NSObject<UIActivityItemSource>
 
 // Default initializer. |text| must not be nil.
 - (instancetype)initWithText:(NSString*)text;
 
 @end
 
 // Returns an image to the UIActivities that can take advantage of it.
 @interface UIActivityImageSource : NSObject<UIActivityItemSource>
 
 // Default initializer. |image| must not be nil.
 - (instancetype)initWithImage:(UIImage*)image;
 
 @end
 
 // This UIActivityItemSource-conforming object communicates with Password
 // Management App Extensions by returning a NSDictionary with the URL of the
 // current page *and* also conforms to UTType public.url so it can be used
 // with other Social Sharing Extensions as well. The |subject| is used by
-// Mail applications to pre-fill in the subject line.
+// Mail applications to pre-fill in the subject line. The |thumbnailGenerator|
+// is used by extensions displaying thumbnails of what is shared, for example

[Olivier, 2017/01/20 10:14:00]

It is not really used by extension, right?
It is used by Chrome when extension request a thumbnail.

[jif, 2017/01/24 10:29:50]

Done.

+// the Chrome iOS Share Extension.
 @interface UIActivityFindLoginActionSource : NSObject<UIActivityItemSource>
 
 // Default initializer. |subject| and |url| must not be nil.
-- (instancetype)initWithURL:(NSURL*)url subject:(NSString*)subject;
+// |thumbnailGenerator| may be nil.
+- (instancetype)initWithURL:(NSURL*)url
+                    subject:(NSString*)subject
+         thumbnailGenerator:(ThumbnailGenerator)thumbnailGenerator;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_ACTIVITY_SERVICES_CHROME_ACTIVITY_ITEM_SOURCE_H_