* Pipe webGL compatibility down to allow attribute aliasing.
* Reject aliasing if shader level is too high, or webGL is in use.

Bug: angleproject:3252
Change-Id: I3378fe707e3795a4b8a424989afccff42bc84344
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1556538Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>