Direct Sockets: ensure script context valid before reading

If the script context is not valid during an attempted read, we exit
early. This avoids crashes, for example, if navigation or page refresh
occurs while a read is pending.

Bug: 905818
Change-Id: I1cf2e44eabb6ce35864c3a95e5930ae15c2cc49e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612231Reviewed-by: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840514}

third_party/blink/renderer/modules/direct_sockets/tcp_readable_stream_wrapper.cc

@@ -142,6 +142,9 @@ void TCPReadableStreamWrapper::OnPeerClosed(MojoResult result,
 }
 
 void TCPReadableStreamWrapper::ReadFromPipeAndEnqueue() {
+  if (!script_state_->ContextIsValid())
+    return;
+
   DVLOG(1) << "TCPReadableStreamWrapper::ReadFromPipeAndEnqueue() this=" << this
            << " in_two_phase_read_=" << in_two_phase_read_
            << " read_pending_=" << read_pending_;