Refactor creation of SadTabView into a tab helper object

ios/chrome/browser/web/sad_tab_tab_helper_delegate.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_CHROME_BROWSER_WEB_SAD_TAB_TAB_HELPER_DELEGATE_H_

[Eugene But (OOO till 7-30), 2017/04/12 01:01:32]

IOS_CHROME_BROWSER_WEB_SAD_TAB_TAB_HELPER_DELEGATE_H_ no need for trailing
underscores.

|tools/boilerplate.py <relative path to source file>| creates a header with
correct guards automatically. 

[peterlaurens, 2017/04/12 17:07:38]

This is the boilerplate boilerplate.py generated, I think I must have used it
wrong! I've removed the prefixed underscores, thanks!

+#define __IOS_CHROME_BROWSER_WEB_SAD_TAB_TAB_HELPER_DELEGATE_H_
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+class SadTabTabHelper;
+
+// SadTabTabHelperDelegate defines an interface that allows SadTabTabHelper
+// instances to learn about the visibility of the Tab they are helping.
+@protocol SadTabTabHelperDelegate<NSObject>
+
+// Returns whether the tab for the tab helper is visible - i.e. the tab is
+// frontmost and the application is in an active state - allowing differences
+// in the behavior of the helper for visible vs. non-visible tabs.
+- (BOOL)isTabVisibleForTabHelper:(SadTabTabHelper*)tabHelper;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif  // __IOS_CHROME_BROWSER_WEB_SAD_TAB_TAB_HELPER_DELEGATE_H_