Framebuffer: Fix getDepthStencilAttachment check.
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 <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ecoal95/angle,ecoal95/angle,ecoal95/angle,MSOpenTech/angle,MSOpenTech/angle,MSOpenTech/angle,ppy/angle,MSOpenTech/angle,ppy/angle,ppy/angle,ecoal95/angle,ecoal95/angle