Add a common base function for getSurfaceLevel in TextureStorage9. Instead of casting to the specific TextureStorage9 type before calling getSurfaceLevel, merge the prototypes into a pure method in TextureStorage9. BUG=angleproject:970 Change-Id: I07b24b6cc470fc8cb6ba9bd9a8f178c3b43be36a Reviewed-on: https://chromium-review.googlesource.com/293270 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>