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/extension_nacl_browsertest.cc b/chrome/browser/extensions/extension_nacl_browsertest.cc
index 8565d8d9..58ccfbb3 100644
--- a/chrome/browser/extensions/extension_nacl_browsertest.cc
+++ b/chrome/browser/extensions/extension_nacl_browsertest.cc
@@ -22,6 +22,7 @@
 
 using content::PluginService;
 using content::WebContents;
+using extensions::Extension;
 
 namespace {