Revert to system Mail app if current default Mail app is no longer there

ios/chrome/browser/web/mailto_url_rewriter.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_MAILTO_URL_REWRITER_H_
 #define IOS_CHROME_BROWSER_WEB_MAILTO_URL_REWRITER_H_
 
 #import <Foundation/Foundation.h>
 
 @class MailtoHandler;
 @class MailtoURLRewriter;
 class GURL;
 
 // Protocol that must be implemented by observers of MailtoURLRewriter object.
 @protocol MailtoURLRewriterObserver<NSObject>
 // The default mailto: handler has been changed.
 - (void)rewriterDidChange:(MailtoURLRewriter*)rewriter;
 @end
 
 // An object that manages the available Mail client apps. The currently selected
 // Mail client to handle mailto: URL is stored in a key in NSUserDefaults.
-// If the key in NSUserDefaults is not set, the system-provided Mail client app
-// will be used.
+// If the key in NSUserDefaults is not set or the corresponding app is no longer
+// installed, the system-provided Mail client app will be used.
 @interface MailtoURLRewriter : NSObject
 
-// The unique ID of the Mail client app was set to handle mailto: URL scheme.
+// The unique ID of the Mail client app that handles mailto: URL scheme.
 @property(nonatomic, copy) NSString* defaultHandlerID;
 
 // Observer object that will be called when |defaultHandlerID| is changed.
 @property(nonatomic, weak) id<MailtoURLRewriterObserver> observer;
 
 // Returns the ID as a string for the system-provided Mail client app.
 + (NSString*)systemMailApp;
 
 // An initializer returning an instance that has the standard set of
 // MailtoHandlers initialized. Unit tests can use -init and then set up the
 // different handlers.
 - (instancetype)initWithStandardHandlers;
 
 // Returns a sorted array of all the currently supported Mail client apps that
 // claim to handle mailto: URL scheme through their own custom defined URL
 // schemes.
 - (NSArray<MailtoHandler*>*)defaultHandlers;
 
 // Returns the name of the application that handles mailto: URLs.
 - (NSString*)defaultHandlerName;
 
 // Rewrites |gURL| into a new URL that can be "opened" to launch the Mail
 // client app. May return nil if |gURL| is not a mailto: URL or there are no
 // Mail client app available.
 - (NSString*)rewriteMailtoURL:(const GURL&)gURL;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_WEB_MAILTO_URL_REWRITER_H_