Adds mailto: URL support to app launching.

ios/chrome/browser/web/mailto_handler_system_mail.h

Index: ios/chrome/browser/web/mailto_handler_system_mail.h
diff --git a/ios/chrome/browser/web/mailto_handler_system_mail.h b/ios/chrome/browser/web/mailto_handler_system_mail.h
new file mode 100644
index 0000000000000000000000000000000000000000..534fd6d217af61c6a3e727a4a483597888f46710
--- /dev/null
+++ b/ios/chrome/browser/web/mailto_handler_system_mail.h
@@ -0,0 +1,14 @@
+// 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_HANDLER_SYSTEM_MAIL_H_
+#define _IOS_CHROME_BROWSER_WEB_MAILTO_HANDLER_SYSTEM_MAIL_H_
+
+#import "ios/chrome/browser/web/mailto_handler.h"
+
+// Handler for Apple Mail client.
+@interface MailtoHandlerSystemMail : MailtoHandler

[rohitrao (ping after 24h), 2017/05/06 00:09:41]

Why do we maintain a handler for apple mail?  Won't the system default to apple
mail if we leave the URL untouched?

[pkl (ping after 24h if needed), 2017/05/08 17:07:55]

In theory, we should be able to leave the URL alone. But treating system Mail in
the same way as 3rd party mail clients eliminates the need to special case the
UI implementation of in Settings (coming up next) for system-provided Mail. It
is "just one of the mail clients".

+@end
+
+#endif  // _IOS_CHROME_BROWSER_WEB_MAILTO_HANDLER_SYSTEM_MAIL_H_