blink_perf.webgpu: Update "vertex_idx" -> "vertex_index"

This follows the deprecation of the old vertex_idx identifier in the
WGSL spec.

Bug: none
Change-Id: If3cf3d3b985b79b85b9807b17e5458bacd0805dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643238
Auto-Submit: Austin Eng <enga@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845773}

third_party/blink/perf_tests/webgpu/binding-draw.html

@@ -38,7 +38,7 @@
                   vec2<f32>(0.5, -0.5));
 
               [[builtin(position)]] var<out> Position : vec4<f32>;
-              [[builtin(vertex_idx)]] var<in> VertexIndex : u32;
+              [[builtin(vertex_index)]] var<in> VertexIndex : u32;
               [[stage(vertex)]] fn main() -> void {
                 Position = vec4<f32>(pos[VertexIndex], 0.0, 1.0);
               }`