Do not produce module code cache on destoryed contexts

Since we produce code cache asynchronously, content might
already be invalid, e.g. due to frame detach.

Added a test which catches the issue in 2 of 10 runs.

Bug: none
Change-Id: Ie4368b2a9dce68d236a7310900488056a9e20765
Reviewed-on: https://chromium-review.googlesource.com/c/1483365Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635153}

third_party/blink/renderer/core/script/modulator_impl_base.cc

@@ -263,6 +263,10 @@ ModulatorImplBase::ModuleRequestsFromScriptModule(ScriptModule script_module) {
 void ModulatorImplBase::ProduceCacheModuleTreeTopLevel(
     ModuleScript* module_script) {
   DCHECK(module_script);
+  // Since we run this asynchronously, context might be gone already,
+  // for example because the frame was detached.
+  if (!script_state_->ContextIsValid())
+    return;
   HeapHashSet<Member<const ModuleScript>> discovered_set;
   ProduceCacheModuleTree(module_script, &discovered_set);
 }

third_party/blink/web_tests/fast/loader/module-no-crash-expected.txt

+ Tests that storing module cache does not crash with detached frame. Note that crash does only reproduce in ~20% of the runs, depending on timing.

third_party/blink/web_tests/fast/loader/module-no-crash.html

+<script>
+if (window.testRunner)
+  testRunner.dumpAsText();
+</script>
+<iframe srcdoc='
+<script type="module">
+if (!localStorage["step"]) {
+  localStorage["step"] = "reload1";
+  window.parent.location.reload();
+} else if (localStorage["step"] === "reload1") {
+  localStorage["step"] = "reload2";
+  window.parent.location.reload();
+} else {
+  window.parent.document.querySelector("iframe").remove();
+}
+</script>
+'></iframe>
+Tests that storing module cache does not crash with detached frame.
+Note that crash does only reproduce in ~20% of the runs, depending on timing.