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 c972efa71f7ad8c2bedb93fcd8ebc8f5416166e9..67c9471e6d9d9401f230ff96f70840967ea45431 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26094,6 +26094,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="MailtoURLRewriter.Rewritten">

[Alexei Svitkine (slow), 2017/05/08 14:56:01]

Please associate this histogram with an enum. e.g. enum="Rewritten" - create it
if it doesn't exist.

[pkl (ping after 24h if needed), 2017/05/08 17:07:56]

I think enum="Boolean" is rather clear given the rename of
IOS.MailtoURLRewritten.
Let me know if there are other advantages of defining an enum that's "Rewritten"
/ "Not Rewritten" (over "True" and "False").

+  <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>

[Alexei Svitkine (slow), 2017/05/08 14:56:01]

We don't recommend adding a new top-level prefix if it won't be used for a lot
metrics. Can this be nested under an existing prefix? Maybe under IOS.?

[pkl (ping after 24h if needed), 2017/05/08 17:07:56]

Renamed to IOS.MailtoURLRewritten.

+
 <histogram name="ManagedUsers.ChromeOS.PasswordChange"
     enum="ManagedUserPasswordChange">
   <owner>achuith@chromium.org</owner>