Move resource storage to attachment base. We can use GetAs() to retrieve the corresponding API object. BUG=angleproject:963 Change-Id: I472e13664e45f9f05621dea3d28135d4e833fb3d Reviewed-on: https://chromium-review.googlesource.com/263485 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>