clang/win: Let remaining chromium_code targets build with -Wextra.

win8/metro_driver/file_picker_ash.h

Index: win8/metro_driver/file_picker_ash.h
diff --git a/win8/metro_driver/file_picker_ash.h b/win8/metro_driver/file_picker_ash.h
index 0862b69663589609313c03b1bc67cb33ee69c48d..c08d293534b6f81596c14466d508c3483e34abde 100644
--- a/win8/metro_driver/file_picker_ash.h
+++ b/win8/metro_driver/file_picker_ash.h
@@ -83,9 +83,7 @@ class OpenFilePickerSession : public FilePickerSessionBase {
     return filenames_;
   }
 
-  const bool allow_multi_select() const {
-    return allow_multi_select_;
-  }
+  bool allow_multi_select() const { return allow_multi_select_; }
 
  private:
   HRESULT StartFilePicker() override;