Adds MailtoURLRewriterObserver to MailtoURLRewriter

ios/chrome/browser/web/mailto_url_rewriter.h

Index: ios/chrome/browser/web/mailto_url_rewriter.h
diff --git a/ios/chrome/browser/web/mailto_url_rewriter.h b/ios/chrome/browser/web/mailto_url_rewriter.h
index d5deb1cdc206713e8a5c0000b199162f8dbf1732..6eb0d865128d282375778c6f80cefa9d867189b9 100644
--- a/ios/chrome/browser/web/mailto_url_rewriter.h
+++ b/ios/chrome/browser/web/mailto_url_rewriter.h
@@ -8,8 +8,15 @@
 #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
@@ -19,6 +26,9 @@ class GURL;
 // The unique ID of the Mail client app was set to handle 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;
 
@@ -31,6 +41,9 @@ class GURL;
 // 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.