Adds mailto: URL support to app launching.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 45067c84ce74a7a239984b50c5735b4f3f1ead78..2e7d45c771ca4a4eeb6721b6ab9ea0b9eb2c93b1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26019,6 +26019,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="MailtoURLRewriter.Rewritten">

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

Should we consider having an enum?
In the futur the enum could be:

-Not rewritten
-Rewritten to Gmail
-Rewritten to Outlook.

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

I set it up this way so there's no question about why we need to know which
non-System Mail client was used.

+  <owner>pkl@chromium.org</owner>
+  <summary>
+    Keeps track of the times that a user tapped on a mailto: link and whether
+    the URL has been rewritten for an available native Mail client app or not.
+  </summary>
+</histogram>
+
 <histogram name="ManagedUsers.ChromeOS.PasswordChange"
     enum="ManagedUserPasswordChange">
   <owner>achuith@chromium.org</owner>