Add missing url/gurl.h include

Functions declared in this file return GURL by value.

BUG=

Review-Url: https://codereview.chromium.org/2627883003
Cr-Commit-Position: refs/heads/master@{#443187}

extensions/common/extension_urls.h

@@ -8,8 +8,7 @@
 #include <string>
 
 #include "base/strings/string16.h"
-
-class GURL;
+#include "url/gurl.h"
 
 namespace extensions {