Fix compile error of brave_browser_tests target (#127)

* Fix compile error of brave_browser_tests target

Add missing base/strings/utf_string_conversions.h header.

* Update brave_extension_provider_browsertest.cc

extensions/browser/brave_extension_provider_browsertest.cc

@@ -3,6 +3,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "base/path_service.h"
+#include "base/strings/utf_string_conversions.h"
 #include "base/test/thread_test_helper.h"
 #include "brave/browser/brave_browser_process_impl.h"
 #include "brave/common/brave_paths.h"