Unused member warning:

..\..\third_party\angle\src\libANGLE/renderer/d3d/d3d11/NativeWindow.h(68,24) :  error: private field 'mConfig' is not used [-Werror,-Wunused-private-field]
    const egl::Config *mConfig;
                       ^
BUG=None

This reverts commit 98233372.

Change-Id: I8ed41770eb3fe70ab7e32ba9a0860e2c397e22c5
Reviewed-on: https://chromium-review.googlesource.com/298830Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>