Use mTopLevel in getSubresourceIndex. We need to include this in every call, so might as well handle it automatically. BUG=angle:729 Change-Id: I5c230408a9eb80c8cfcc3f0d3a4ea531fa173aed Reviewed-on: https://chromium-review.googlesource.com/221760 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>