function isInIFrame(w) {
      w = w || topWindow;
      return w !== w.top;
    }
Loads a shader.
@param {WebGLRenderingContext} gl The WebGLRenderingContext to use.
@param {string} shaderSource The shader source.
@param {number} shaderType The type of shader.
@param {module:webgl-utils.ErrorCallback} opt_errorCallback callback for errors.
@return {WebGLShader} The created shader.