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/mul/mat2x4-mat4x2/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   mat2x4 a = mat2x4(vec4(1.0f, 2.0f, 3.0f, 4.0f), vec4(5.0f, 6.0f, 7.0f, 8.0f));

test/expressions/binary/mul/mat3x3-mat3x3/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   mat3 a = mat3(vec3(1.0f, 2.0f, 3.0f), vec3(4.0f, 5.0f, 6.0f), vec3(7.0f, 8.0f, 9.0f));

test/expressions/binary/mul/mat4x2-mat2x4/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   mat4x2 a = mat4x2(vec2(-1.0f, -2.0f), vec2(-3.0f, -4.0f), vec2(-5.0f, -6.0f), vec2(-7.0f, -8.0f));

test/expressions/binary/mul/scalar-scalar/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   float r = (1.0f * 2.0f);

test/expressions/binary/mul/scalar-scalar/i32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   int r = (1 * 2);

test/expressions/binary/mul/scalar-scalar/u32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   uint r = (1u * 2u);

test/expressions/binary/mul/scalar-vec3/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   float a = 4.0f;

test/expressions/binary/mul/scalar-vec3/i32.wgsl.expected.glsl

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

test/expressions/binary/mul/scalar-vec3/u32.wgsl.expected.glsl

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

test/expressions/binary/mul/vec3-scalar/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   vec3 a = vec3(1.0f, 2.0f, 3.0f);

test/expressions/binary/mul/vec3-scalar/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/mul/vec3-scalar/u32.wgsl.expected.glsl

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

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

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void f() {
   vec3 a = vec3(1.0f, 2.0f, 3.0f);

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

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