Framebuffer: Fix getDepthStencilAttachment check. The check should be using the newer operator== to check if attachments are the same - the id() method will hit an UNREACHABLE if it's called on a Surface attachment. I noticed this when using this method in the Vulkan RenderPass implementation. BUG=angleproject:1583 Change-Id: I387b35805893d04003b22a1c7c9c12fc41c726ce Reviewed-on: https://chromium-review.googlesource.com/412801 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>