[WebSocket] Fix a crash caused by a pending empty frame.

net/websockets/websocket_channel.cc

Index: net/websockets/websocket_channel.cc
diff --git a/net/websockets/websocket_channel.cc b/net/websockets/websocket_channel.cc
index 7efc5709d7fe61d8ad1e9003b84696330871aaf3..624f38f3ea1543aeeb2e2d8173d24a0e33d68df6 100644
--- a/net/websockets/websocket_channel.cc
+++ b/net/websockets/websocket_channel.cc
@@ -407,7 +407,9 @@ void WebSocketChannel::SendFlowControl(int64 quota) {
     const size_t bytes_to_send =
         std::min(base::checked_cast<size_t>(quota), data_size);
     const bool final = front.final() && data_size == bytes_to_send;
-    const char* data = front.data()->data() + front.offset();
+    const char* data = front.data() ?
+        front.data()->data() + front.offset() : NULL;
+    DCHECK(!bytes_to_send || data) << "Non empty data should not be null.";
     const std::vector<char> data_vector(data, data + bytes_to_send);
     DVLOG(3) << "Sending frame previously split due to quota to the "
              << "renderer: quota=" << quota << " data_size=" << data_size