Adds mailto: URL support to app launching.

ios/chrome/browser/web/mailto_handler_gmail.mm

+// 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.
+
+#import "ios/chrome/browser/web//mailto_handler_gmail.h"
+#include "base/strings/sys_string_conversions.h"
+#include "url/gurl.h"
+#include "url/url_constants.h"
+
+#import <UIKit/UIKit.h>
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation MailtoHandlerGmail
+
+- (instancetype)init {
+  // Gmail product name is not supposed to be localized.
+  self = [super initWithName:@"Gmail" appStoreID:@"422689480"];
+  return self;
+}
+
+- (BOOL)isAvailable {
+  // Checks if Gmail app is installed on this device. This assumes that
+  // "googlegmail" has been whitelisted in Chrome's Info.plist.
+  return [[UIApplication sharedApplication]
+      canOpenURL:[NSURL URLWithString:@"googlegmail://"]];
+}
+
+- (NSString*)rewriteMailtoURL:(const GURL&)gURL {

[jif, 2017/05/03 13:01:35]

Consider creating a function in the same vein as
https://github.com/mozilla-mobile/firefox-ios/blob/0da53b909b73ae61b480f353b7...

This will make the implementation of |rewriteMailtoURL:| for the other mail
clients simpler.

[pkl (ping after 24h if needed), 2017/05/03 22:17:18]

Instead, I moved this up to the base class so subclasses can just inherit the
implementation (if they wish).
I've also added -beginningScheme and -supportedHeaders.

+  if (!gURL.SchemeIs(url::kMailToScheme))
+    return nil;
+  NSString* recipient = base::SysUTF8ToNSString(gURL.path());
+  NSString* query = base::SysUTF8ToNSString(gURL.query());
+  return
+      [NSString stringWithFormat:@"googlegmail:/co?to=%@&%@", recipient, query];
+}
+
+@end