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/buffer/uniform/dynamic_index/read.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct Inner {
   ivec3 a;

test/buffer/uniform/static_index/read.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct Inner {
   int x;

test/buffer/uniform/types/array.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   vec4 inner[4];

test/buffer/uniform/types/f32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   float inner;

test/buffer/uniform/types/i32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   int inner;

test/buffer/uniform/types/mat2x2.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   mat2 inner;

test/buffer/uniform/types/mat2x3.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   mat2x3 inner;

test/buffer/uniform/types/mat3x2.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   mat3x2 inner;

test/buffer/uniform/types/mat4x4.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   mat4 inner;

test/buffer/uniform/types/struct.wgsl.expected.glsl

@@ -1,7 +1,6 @@
 SKIP: FAILED
 
 #version 310 es
-precision mediump float;
 
 struct Inner {
   float f;
@@ -25,8 +24,8 @@ void main() {
   return;
 }
 Error parsing GLSL shader:
-ERROR: 0:17: '=' :  cannot convert from 'layout( binding=0 column_major shared) uniform block{layout( column_major shared) uniform structure{ global mediump float f} inner}' to ' temp structure{ global structure{ global mediump float f} inner}'
-ERROR: 0:17: '' : compilation terminated 
+ERROR: 0:16: '=' :  cannot convert from 'layout( binding=0 column_major shared) uniform block{layout( column_major shared) uniform structure{ global highp float f} inner}' to ' temp structure{ global structure{ global highp float f} inner}'
+ERROR: 0:16: '' : compilation terminated 
 ERROR: 2 compilation errors.  No code generated.
 
 

test/buffer/uniform/types/u32.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   uint inner;

test/buffer/uniform/types/vec2.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   ivec2 inner;

test/buffer/uniform/types/vec3.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   uvec3 inner;

test/buffer/uniform/types/vec4.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 struct u_block {
   vec4 inner;