Group ConnectToApplication-related info into a params struct.

mojo/shell/identity.h

Index: mojo/shell/identity.h
diff --git a/mojo/shell/identity.h b/mojo/shell/identity.h
index 1aca15856184659c87034d5837ac159541d4409f..865d65284a22f112bf51297fa011025cb83d5cca 100644
--- a/mojo/shell/identity.h
+++ b/mojo/shell/identity.h
@@ -11,18 +11,23 @@ namespace mojo {
 namespace shell {
 
 /**
- * Represents the identity of an application. |url| is the url of the
+ * Represents the identity of an application. |url| is the URL of the
  * application. |qualifier| is a string that allows to tie a specific instance
- * of an application to another. It is used by content handlers that need to be
- * run in the context of another application.
+ * of an application to another. A typical use case of qualifier is to control
+ * process grouping for a given application URL. For example, the core services
+ * are grouped into "Core"/"Files"/"Network"/etc. using qualifier; content
+ * handler's qualifier is derived from the origin of the content.
  */
 struct Identity {
+  Identity();
   Identity(const GURL& in_url, const std::string& in_qualifier);
   explicit Identity(const GURL& in_url);
+
   bool operator<(const Identity& other) const;
+  bool is_null() const { return url.is_empty(); }
 
-  const GURL url;
-  const std::string qualifier;
+  GURL url;
+  std::string qualifier;
 };
 
 }  // namespace shell