Revert to system Mail app if current default Mail app is no longer there

ios/chrome/browser/web/mailto_url_rewriter.h

Index: ios/chrome/browser/web/mailto_url_rewriter.h
diff --git a/ios/chrome/browser/web/mailto_url_rewriter.h b/ios/chrome/browser/web/mailto_url_rewriter.h
index 1c168a09905e68a83c10985658bf89988f57456c..cb3f515a57a0dd47badddcc8d6da5c0ec56c0182 100644
--- a/ios/chrome/browser/web/mailto_url_rewriter.h
+++ b/ios/chrome/browser/web/mailto_url_rewriter.h
@@ -19,11 +19,11 @@ class GURL;
 
 // An object that manages the available Mail client apps. The currently selected
 // Mail client to handle mailto: URL is stored in a key in NSUserDefaults.
-// If the key in NSUserDefaults is not set, the system-provided Mail client app
-// will be used.
+// If the key in NSUserDefaults is not set or set but is no longer installed,

[rohitrao (ping after 24h), 2017/05/24 11:56:58]

"or the corresponding app is no longer installed"

[pkl (ping after 24h if needed), 2017/05/24 15:53:43]

Done.

+// the system-provided Mail client app will be used.
 @interface MailtoURLRewriter : NSObject
 
-// The unique ID of the Mail client app was set to handle mailto: URL scheme.
+// The unique ID of the Mail client app that handles mailto: URL scheme.
 @property(nonatomic, copy) NSString* defaultHandlerID;
 
 // Observer object that will be called when |defaultHandlerID| is changed.