Add WebGL testing hooks to make it possible to test GraphicsContextGL calls that time out during WebGL operation

void WebGLRenderingContextBase::paintRenderingResultsToCanvas()
{
    if (isContextLostOrPending())
        return;
    ...
        prepareForDisplay();
    ...
                    m_context->paintCompositedResultsToCanvas(*buffer);

}

We'd like to test what happens when timeout happens in prepareForDisplay(), so that paintCompositedResultsToCanvas() is executed on a failing context.