GLSL: only emit default precision qualifier for frag shader.

Other shader types don't need this.
Also fix code style of member var.

Bug: tint:1360
Change-Id: Ic3600ec7c6da9b85b57655fabbf1f2e44b0ea7d3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/79640
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

test/expressions/binary/mod/vec3-vec3/f32.wgsl.expected.glsl

@@ -1,7 +1,6 @@
 SKIP: FAILED
 
 #version 310 es
-precision mediump float;
 
 void f() {
   vec3 a = vec3(1.0f, 2.0f, 3.0f);
@@ -15,8 +14,8 @@ void main() {
   return;
 }
 Error parsing GLSL shader:
-ERROR: 0:7: '%' :  wrong operand types: no operation '%' exists that takes a left-hand operand of type ' temp mediump 3-component vector of float' and a right operand of type ' temp mediump 3-component vector of float' (or there is no acceptable conversion)
-ERROR: 0:7: '' : compilation terminated 
+ERROR: 0:6: '%' :  wrong operand types: no operation '%' exists that takes a left-hand operand of type ' temp highp 3-component vector of float' and a right operand of type ' temp highp 3-component vector of float' (or there is no acceptable conversion)
+ERROR: 0:6: '' : compilation terminated 
 ERROR: 2 compilation errors.  No code generated.
 
 

test/expressions/binary/mod/vec3-vec3/i32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   ivec3 a = ivec3(1, 2, 3);

test/expressions/binary/mod/vec3-vec3/u32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   uvec3 a = uvec3(1u, 2u, 3u);