tools: Exit run-cts when the coverage page is closed

Change-Id: I70f3ffa40dcfe60cdee46bdf0f5db21370be1a3e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113861
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>

tools/src/cov/view-coverage.html

@@ -262,6 +262,10 @@
             readOnly: true,
         });
 
+        addEventListener('beforeunload', () => {
+            fetch("viewer.closed");
+        });
+
         window.onload = function () {
             el_source.doc.setValue("// Loading... ");
             fetch("coverage.dat").then(response =>