I think the patch for bug 105552 might have introduced this memory leak. It happens with qtwebkit at least although with some minor modifications to not pre-allocate the gst buffers from the ResourceHandle I think it would happen in GTK as well.
Workaround: disable the webkit internal http element (see initializeGStreamerAndRegisterWebKitElements)
Turns out this is a qtwebkit bug: https://bugreports.qt-project.org/browse/QTBUG-369793