[Extensions] Pull ExtensionId into its own file

extensions/browser/content_hash_fetcher.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EXTENSIONS_BROWSER_CONTENT_HASH_FETCHER_H_
 #define EXTENSIONS_BROWSER_CONTENT_HASH_FETCHER_H_
 
+#include <map>
 #include <set>
 #include <string>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
-#include "extensions/common/extension.h"
+#include "extensions/common/extension_id.h"
 
 namespace content {
 class BrowserContext;
 }
 
 namespace extensions {
-
+class Extension;
 class ExtensionRegistry;
 class ContentHashFetcherJob;
 class ContentVerifierDelegate;
 
 // This class is responsible for getting signed expected hashes for use in
 // extension content verification. As extensions are loaded it will fetch and
 // parse/validate/cache this data as needed, including calculating expected
 // hashes for each block of each file within an extension. (These unsigned leaf
 // node block level hashes will always be checked at time of use use to make
 // sure they match the signed treehash root hash).
@@ skipping 42 matching lines @@
 
   // Used for binding callbacks passed to jobs.
   base::WeakPtrFactory<ContentHashFetcher> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ContentHashFetcher);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_CONTENT_HASH_FETCHER_H_