SubresourceTrackingPerf: use the result of textureDimensions

This fixes a failure when use_tint_generator on D3D12, but also preps
the code for the future where WGSL requires that return values from
functions are used.

Bug: None
Change-Id: I6b8a5e0aef6550ef39d2972cdc510e05e4be3662
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/46620
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/tests/perf_tests/SubresourceTrackingPerf.cpp

@@ -80,7 +80,7 @@ class SubresourceTrackingPerf : public DawnPerfTestWithParams<SubresourceTrackin
             [[location(0)]] var<out> FragColor : vec4<f32>;
             [[group(0), binding(0)]] var materials : texture_2d<f32>;
             [[stage(fragment)]] fn main() -> void {
-                textureDimensions(materials);
+                const foo : vec2<i32> = textureDimensions(materials);
                 FragColor = vec4<f32>(1.0, 0.0, 0.0, 1.0);
             }
         )");