Adds mailto: URL support to app launching.

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 GURL;
+
+// Class of a singleton object that manages the available Mail client apps
+// and the selected Mail client app to handle mailto: URL scheme.
+@interface MailtoURLRewriter : NSObject
+
+// 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 an array of all the currently supported Mail client apps that claims
+// to handle mailto: URL scheme through their own custom defined URL schemes.
+- (NSArray<MailtoHandler*>*)defaultHandlers;
+
+// Returns the ID as a string for the Mail client app was set to handle mailto:
+// URL scheme.
+- (NSString*)defaultHandlerID;
+
+// Sets the ID as a string for the Mail client app that will handle mailto:
+// URL scheme.
+- (void)setDefaultHandlerID:(NSString*)appStoreID;
+
+// Rewrites |gURL|, which must be a mailto: URL, into a new URL that can be
+// "opened" to launch the Mail client app.

[jif, 2017/05/04 14:08:47]

Can you mention that this can return nil? For example:

Can return nil for a number of reasons, including: no Mail client was selected,
the selected Mail client is not installed anymore, the selected Mail client is
not supported by Chrome anymore.

[pkl (ping after 24h if needed), 2017/05/04 22:25:59]

It's probably unnecessary to enumerate all the possible reasons other than
"there are no Mail client app" available.
I just said the miniumum.

+- (NSString*)rewriteMailtoURL:(const GURL&)gURL;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_WEB_MAILTO_URL_REWRITER_H_