tint/writer/glsl: Set default precision to highp

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>
diff --git a/test/tint/builtins/gen/var/min/03c7e3.wgsl.expected.glsl b/test/tint/builtins/gen/var/min/03c7e3.wgsl.expected.glsl
index 6767d44..9cb7748 100644
--- a/test/tint/builtins/gen/var/min/03c7e3.wgsl.expected.glsl
+++ b/test/tint/builtins/gen/var/min/03c7e3.wgsl.expected.glsl
@@ -25,7 +25,7 @@
   return;
 }
 #version 310 es
-precision mediump float;
+precision highp float;
 
 layout(binding = 0, std430) buffer prevent_dce_block_ssbo {
   ivec2 inner;