Fix memory leak with default Framebuffers.

When the depth-stencil attachment had no depth or stencil bits, we
would like a gl::Renderbuffer object. This was causing unbounded
memory growth in Chrome, and could happen when you watched a lot
of streaming video.

The bug was originally reported and fixed by Austin Kinross in
commit 44f4d74c0f07fa23.

BUG=403471

Change-Id: I022f01a6d3159f766e65f65f023e6ce9baacbc8e
Reviewed-on: https://chromium-review.googlesource.com/231235
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Hendrik Wagenaar <hendrikw@chromium.org>
Tested-by: Hendrik Wagenaar <hendrikw@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>