This is a refactoring change only in preparation for a change in the
Vulkan back-end. We'll need to split the handling for Vulkan and GL
into different functions.

Bug: angleproject:4517
Change-Id: I2ddf9c70fe247d1dcb4e0aa9ca929bbbd61bd42b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2135893Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>