Add the ability to load multiple apps into app_shell.

extensions/shell/browser/shell_extension_system.h

Index: extensions/shell/browser/shell_extension_system.h
diff --git a/extensions/shell/browser/shell_extension_system.h b/extensions/shell/browser/shell_extension_system.h
index 08004cebeb599b1cb7291faa96138399182f1d1c..fbb40ba1b783e45c798e5e1b79d3813fcdcd3f96 100644
--- a/extensions/shell/browser/shell_extension_system.h
+++ b/extensions/shell/browser/shell_extension_system.h
@@ -9,6 +9,7 @@
 
 #include "base/compiler_specific.h"
 #include "extensions/browser/extension_system.h"
+#include "extensions/common/extension_set.h"
 #include "extensions/common/one_shot_event.h"
 
 class BrowserContextKeyedServiceFactory;
@@ -38,17 +39,19 @@ class ShellExtensionSystem : public ExtensionSystem {
   explicit ShellExtensionSystem(content::BrowserContext* browser_context);
   ~ShellExtensionSystem() override;
 
-  // Loads an unpacked application from a directory. Returns true on success.
-  bool LoadApp(const base::FilePath& app_dir);
+  // Loads an unpacked application from a directory. Returns the extension on
+  // success, or null otherwise.
+  const Extension* LoadApp(const base::FilePath& app_dir);
+
+  // Initializes the extension system.
+  bool Init();
 
   // Launch the currently loaded app.
-  void LaunchApp();
+  void LaunchApp(const ExtensionId& extension_id);
 
   // KeyedService implementation:
   void Shutdown() override;
 
-  scoped_refptr<Extension> extension() { return extension_; }
-
   // ExtensionSystem implementation:
   void InitForRegularProfile(bool extensions_enabled) override;
   ExtensionService* extension_service() override;
@@ -81,10 +84,8 @@ class ShellExtensionSystem : public ExtensionSystem {
  private:
   content::BrowserContext* browser_context_;  // Not owned.
 
-  // Extension ID for the app.
-  std::string app_id_;
-
-  scoped_refptr<Extension> extension_;
+  // Set with all loaded apps.
+  ExtensionSet extensions_;
 
   // Data to be accessed on the IO thread. Must outlive process_manager_.
   scoped_refptr<InfoMap> info_map_;