WGSL states that `f32` is a IEEE-754 binary32. mediump precision guarantees are much smaller than highp, and using mediump can break code generated by tint.
Change-Id: I8a129c4c8ac078c785e0d5d7d21f19ed6b3620b5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/123400
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Stephen White <senorblanco@chromium.org>
