[Cronet] Use WeakPtr in CronetBidirectionalStream on iOS.

components/cronet/ios/test/cronet_bidirectional_stream_test.mm

Index: components/cronet/ios/test/cronet_bidirectional_stream_test.mm
diff --git a/components/cronet/ios/test/cronet_bidirectional_stream_test.mm b/components/cronet/ios/test/cronet_bidirectional_stream_test.mm
index 98b44f279c83d8a5e8554d6e829f6d2cd4705ed8..836fac606f234d76968ee608b7878a6f6692f800 100644
--- a/components/cronet/ios/test/cronet_bidirectional_stream_test.mm
+++ b/components/cronet/ios/test/cronet_bidirectional_stream_test.mm
@@ -167,6 +167,8 @@ class TestBidirectionalStreamCallback {
       return false;
 
     cronet_bidirectional_stream_cancel(stream);
+    cronet_bidirectional_stream_write(stream, "abc", 3, false);
+
     return true;
   }