CTS: Allow sync code to interrupt and cancel a scheduled heartbeat

Bug: chromium:1340602
Change-Id: Id6f8d58bd283c8232af533cf2a0fe74adb704336
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/102180
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

webgpu-cts/test_runner.js

@@ -37,9 +37,8 @@ function rateLimited(fn, intervalMs) {
   let last = undefined;
   let timer = undefined;
   const wrappedFn = (...args) => {
-    if (timer !== undefined || last === undefined) {
-      // If there is already a fn call scheduled, or the function is
-      // not enabled, return.
+    if (last === undefined) {
+      // If the function is not enabled, return.
       return;
     }
     // Get the current time as a number.