Adds mailto: URL support to app launching.

ios/chrome/browser/web/external_app_launcher.mm

Index: ios/chrome/browser/web/external_app_launcher.mm
diff --git a/ios/chrome/browser/web/external_app_launcher.mm b/ios/chrome/browser/web/external_app_launcher.mm
index 9489a55ca058771bbbdef29a9697749fa95f123d..f3c944fa830764ceff6c0d634b14c08dcd1e6151 100644
--- a/ios/chrome/browser/web/external_app_launcher.mm
+++ b/ios/chrome/browser/web/external_app_launcher.mm
@@ -11,10 +11,12 @@
 #include "components/strings/grit/components_strings.h"
 #include "ios/chrome/browser/experimental_flags.h"
 #import "ios/chrome/browser/open_url_util.h"
+#import "ios/chrome/browser/web/mailto_url_rewriter.h"
 #include "ios/chrome/grit/ios_strings.h"
 #import "net/base/mac/url_conversions.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "url/gurl.h"
+#include "url/url_constants.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
@@ -52,7 +54,7 @@ void RecordExternalApplicationOpened(bool opened) {
 }
 
 // Returns whether gURL has the scheme of a URL that initiates a call.
-BOOL UrlHasPhoneCallScheme(const GURL& gURL) {
+bool UrlHasPhoneCallScheme(const GURL& gURL) {
   return gURL.SchemeIs("tel") || gURL.SchemeIs("facetime") ||
          gURL.SchemeIs("facetime-audio");
 }
@@ -186,6 +188,19 @@ NSString* PromptActionString(NSString* scheme) {
     }
   }
 
+  // Replaces |URL| with a rewritten URL if it is of mailto: scheme.
+  if (!experimental_flags::IsNativeAppLauncherEnabled() &&
+      gURL.SchemeIs(url::kMailToScheme)) {
+    NSString* launchURL =
+        [[MailtoURLRewriter sharedInstance] rewriteMailtoURL:gURL];

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

MailtoURLRewriter doesn't have to be a singleton.

You can:
* have ExternalAppLauncher own it (not recommended, likely premature
optimization).
* recreate it every time (recommended).

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

Given how ExternalAppLauncher is used, it is likely that MailtoURLRewriter will
be recreated every time. May as well keep it simple and just recreate it when it
is needed.

+    if (launchURL) {
+      UMA_HISTOGRAM_BOOLEAN("MailtoURLRewriter.Rewritten", true);
+      URL = [NSURL URLWithString:launchURL];
+    } else {
+      UMA_HISTOGRAM_BOOLEAN("MailtoURLRewriter.Rewritten", false);
+    }
+  }
+
   // If the following call returns YES, an external application is about to be
   // launched and Chrome will go into the background now.
   // TODO(crbug.com/622735): This call still needs to be updated.