mash: Replace int ShelfIDs with AppLaunchID strings.

ash/public/cpp/app_launch_id.h

Index: ash/public/cpp/app_launch_id.h
diff --git a/ash/public/cpp/app_launch_id.h b/ash/public/cpp/app_launch_id.h
index 1b080ec5534db6ea075463734286eea6eaca061c..30f184d3d36e8c9c84fe3428a49955f2c1ccbc3c 100644
--- a/ash/public/cpp/app_launch_id.h
+++ b/ash/public/cpp/app_launch_id.h
@@ -25,9 +25,15 @@ class ASH_PUBLIC_EXPORT AppLaunchId {
   AppLaunchId();
   ~AppLaunchId();
 
-  AppLaunchId(const AppLaunchId& app_launch_id) = default;
-  AppLaunchId(AppLaunchId&& app_launch_id) = default;
+  AppLaunchId(const AppLaunchId& other) = default;
+  AppLaunchId(AppLaunchId&& other) = default;
   AppLaunchId& operator=(const AppLaunchId& other) = default;
+  bool operator==(const AppLaunchId& other) const;
+  bool operator!=(const AppLaunchId& other) const;
+  bool operator<(const AppLaunchId& other) const;
+
+  // Returns true if both the application id and launch id are empty.
+  bool IsEmpty() const;

[James Cook, 2017/05/04 16:38:48]

naming question: Should this be IsValid()? Or have the comment say something
about what it means when both are empty? It's not clear from just this header
what empty is used for.

[msw, 2017/05/04 19:05:56]

Hmm, it would be nice to invert the boolean, but I didn't want to claim that an
ID is "valid" simply because one or both of the strings are not empty. Validity
might seem to imply more than that. It'd change many callsites, but I'll switch
to IsValid or maybe IsViable or similar if you prefer that (left as-is for now).

[James Cook, 2017/05/04 19:45:07]

OK. Please update the comment to say that empty is often used as a null or
invalid value.

highly optional: What do you think about IsNull() vs. IsEmpty()?

[msw, 2017/05/04 20:39:07]

Done.

 
   const std::string& app_id() const { return app_id_; }
   const std::string& launch_id() const { return launch_id_; }