Add a RefCountObject::getRefCount method for testing. BUG=angleproject:970 Change-Id: I4fae29766ee76512375872535dbaf2d31a379de0 Reviewed-on: https://chromium-review.googlesource.com/286551 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>