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/samples/compute_boids.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(location = 0) in vec2 a_particlePos_1;
 layout(location = 1) in vec2 a_particleVel_1;
@@ -69,7 +68,6 @@ void main() {
   return;
 }
 #version 310 es
-precision mediump float;
 
 struct Particle {
   vec2 pos;

test/samples/cube.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 layout(location = 0) in vec4 cur_position_1;
 layout(location = 1) in vec4 color_1;

test/samples/function.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 void ep() {
 }

test/samples/simple_vertex.spvasm.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 vec4 tint_symbol = vec4(0.0f, 0.0f, 0.0f, 0.0f);
 void main_1() {

test/samples/triangle.wgsl.expected.glsl

@@ -1,5 +1,4 @@
 #version 310 es
-precision mediump float;
 
 const vec2 pos[3] = vec2[3](vec2(0.0f, 0.5f), vec2(-0.5f, -0.5f), vec2(0.5f, -0.5f));
 vec4 vtx_main(uint VertexIndex) {