Move Extension into extensions namespace

BUG=117262
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Review URL: https://chromiumcodereview.appspot.com/10375021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138074 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/extensions/unpacked_installer.h b/chrome/browser/extensions/unpacked_installer.h
index 437e2235..ad60d0a 100644
--- a/chrome/browser/extensions/unpacked_installer.h
+++ b/chrome/browser/extensions/unpacked_installer.h
@@ -12,11 +12,12 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 
-class Extension;
 class ExtensionService;
 
 namespace extensions {
 
+class Extension;
+
 // Installs and loads an unpacked extension.
 // TODO(erikkay): It might be useful to be able to load a packed extension
 // (presumably into memory) without installing it.