Fix a flaky QUIC end_to_end_test when the socket was suddenly unwritable

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 33c489a479e9bf9d3d485f8600f62e3dbe4b7be2..47361109c6a924535275644f233353e0bcb29978 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -1290,6 +1290,10 @@ bool QuicConnection::ShouldGeneratePacket(
 }
 
 bool QuicConnection::CanWrite(HasRetransmittableData retransmittable) {
+  if (!connected_) {
+    return false;
+  }
+
   if (writer_->IsWriteBlocked()) {
     visitor_->OnWriteBlocked();
     return false;