[Extensions] Pull ExtensionId into its own file

extensions/browser/content_hash_fetcher.h

Index: extensions/browser/content_hash_fetcher.h
diff --git a/extensions/browser/content_hash_fetcher.h b/extensions/browser/content_hash_fetcher.h
index 7367ca3992c03571ba70f1d78e79a14b1a5813e1..f6e243876d1bee92b013e04645da86965b3580a8 100644
--- a/extensions/browser/content_hash_fetcher.h
+++ b/extensions/browser/content_hash_fetcher.h
@@ -5,6 +5,7 @@
 #ifndef EXTENSIONS_BROWSER_CONTENT_HASH_FETCHER_H_
 #define EXTENSIONS_BROWSER_CONTENT_HASH_FETCHER_H_
 
+#include <map>
 #include <set>
 #include <string>
 
@@ -12,14 +13,14 @@
 #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;