Transferable Streams: Don't permit the remote side to crash us

The transferable streams implementation assumed that two kPull messages
couldn't be sent without an intervening chunk. This wasn't true in the
case of a compromised renderer, and since
https://chromium-review.googlesource.com/c/chromium/src/+/2291180 hasn't
been true in general.

This change aligns the implementation with the proposed standard at
https://chromium-review.googlesource.com/c/chromium/src/+/2291180.

This fixes flakiness in the http/tests/streams/transferable/worker.html
test.

BUG=1106239,1106239

Change-Id: I12c9d011c4a42c8178fcaffa4ddd23e38fb82d9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303329Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789389}

third_party/blink/renderer/core/streams/transferable_streams.cc

@@ -454,9 +454,10 @@ void CrossRealmTransformWritable::HandleMessage(MessageType type,
                                                 v8::Local<v8::Value> value) {
   switch (type) {
     case MessageType::kPull:
-      DCHECK(backpressure_promise_);
-      backpressure_promise_->ResolveWithUndefined(script_state_);
-      backpressure_promise_ = nullptr;
+      if (backpressure_promise_) {
+        backpressure_promise_->ResolveWithUndefined(script_state_);
+        backpressure_promise_ = nullptr;
+      }
       return;
 
     case MessageType::kCancel: