[wasm] Notify StreamingDecoder when its AsyncCompileJob is destructed

The lifetime of the AsyncCompileJob does not depend on the lifetime of
the stream which feeds data into it. Multiple checks guarantee that the
AsyncCompileJob still exists when the stream wants to call it. With
this CL we add an additional level of defense to make sure that
streaming does not continue after the AsyncCompileJob got destructed.

It is not clear if this CL fixes the bug referenced below. However, the
crashes there could be caused when streaming accesses the
AsyncCompileJob after it got destructed already. I was not able though
to find a scenario where this is possible.

R=clemensh@chromium.org

Bug: chromium:888170
Change-Id: Id5c6cc34842735a3adaf3e09c57cbe923cfc2630
Reviewed-on: https://chromium-review.googlesource.com/1241961
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56213}

src/wasm/module-compiler.cc

@@ -2254,6 +2254,11 @@ std::shared_ptr<StreamingDecoder> AsyncCompileJob::CreateStreamingDecoder() {
 AsyncCompileJob::~AsyncCompileJob() {
   background_task_manager_.CancelAndWait();
   if (native_module_) native_module_->compilation_state()->Abort();
+  // Tell the streaming decoder that the AsyncCompileJob is not available
+  // anymore.
+  // TODO(ahaas): Is this notification really necessary? Check
+  // https://crbug.com/888170.
+  if (stream_) stream_->NotifyCompilationEnded();
   CancelPendingForegroundTask();
   for (auto d : deferred_handles_) delete d;
 }
@@ -2285,7 +2290,6 @@ void AsyncCompileJob::FinishCompile() {
 }
 
 void AsyncCompileJob::AsyncCompileFailed(Handle<Object> error_reason) {
-  if (stream_) stream_->NotifyError();
   // {job} keeps the {this} pointer alive.
   std::shared_ptr<AsyncCompileJob> job =
       isolate_->wasm_engine()->RemoveCompileJob(this);

src/wasm/streaming-decoder.h

@@ -65,8 +65,13 @@ class V8_EXPORT_PRIVATE StreamingDecoder {
 
   void Abort();
 
-  // Notify the StreamingDecoder that there has been an compilation error.
-  void NotifyError() { ok_ = false; }
+  // Notify the StreamingDecoder that compilation ended and the
+  // StreamingProcessor should not be called anymore.
+  void NotifyCompilationEnded() {
+    // We set {ok_} to false to turn all future calls to the StreamingDecoder
+    // into no-ops.
+    ok_ = false;
+  }
 
  private:
   // TODO(ahaas): Put the whole private state of the StreamingDecoder into the