Allow attribute linking for ES2 * 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/+/1556538 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>