[Extensions] Pull ExtensionId into its own file

chrome/browser/background/background_mode_manager.h

Index: chrome/browser/background/background_mode_manager.h
diff --git a/chrome/browser/background/background_mode_manager.h b/chrome/browser/background/background_mode_manager.h
index 986863df5d5bf078f00594fc4f26283e9cd4bbe3..8bfabe87d6ee5fdc80119c82dcca6387d5fafd12 100644
--- a/chrome/browser/background/background_mode_manager.h
+++ b/chrome/browser/background/background_mode_manager.h
@@ -24,7 +24,7 @@
 #include "components/prefs/pref_change_registrar.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
-#include "extensions/common/extension.h"
+#include "extensions/common/extension_id.h"
 
 class BackgroundTrigger;
 class Browser;
@@ -37,6 +37,10 @@ namespace base {
 class CommandLine;
 }
 
+namespace extensions {
+class Extension;
+}
+
 typedef std::vector<base::Closure> CommandIdHandlerVector;
 
 // BackgroundModeManager is responsible for switching Chrome into and out of