[Extensions] Pull ExtensionId into its own file

extensions/common/extension.h

Index: extensions/common/extension.h
diff --git a/extensions/common/extension.h b/extensions/common/extension.h
index fc6579e18f596ab8dfc6037e97ac75a8c7607ed1..1cc693ea2fb70db207b2ea2dbb4c8c94e52fe983 100644
--- a/extensions/common/extension.h
+++ b/extensions/common/extension.h
@@ -16,6 +16,7 @@
 #include "base/memory/linked_ptr.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/thread_checker.h"
+#include "extensions/common/extension_id.h"
 #include "extensions/common/extension_resource.h"
 #include "extensions/common/install_warning.h"
 #include "extensions/common/manifest.h"
@@ -36,13 +37,6 @@ class PermissionSet;
 class PermissionsData;
 class PermissionsParser;
 
-// Uniquely identifies an Extension, using 32 characters from the alphabet
-// 'a'-'p'.  An empty string represents "no extension".
-//
-// Note: If this gets used heavily in files that don't otherwise need to include
-// extension.h, we should pull it into a dedicated header.
-typedef std::string ExtensionId;
-
 // Represents a Chrome extension.
 // Once created, an Extension object is immutable, with the exception of its
 // RuntimeData. This makes it safe to use on any thread, since access to the