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/array/assign_to_function_var.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {

test/array/assign_to_private_var.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {

test/array/assign_to_storage_var.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {

test/array/assign_to_subexpr.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {

test/array/assign_to_workgroup_var.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {

test/array/function_parameter.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 float f1(float a[4]) {
   return a[3];

test/array/function_return_type.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 float[4] f1() {
   float tint_symbol_1[4] = float[4](0.0f, 0.0f, 0.0f, 0.0f);

test/array/strides.spvasm.expected.glsl

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

test/array/type_constructor.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void tint_symbol() {
   int x = 42;