The latest WebKit has changed the function gl.getUniformLocation. This function now returns an object. According to the OpenGL ES 2.0 standard it should return -1 if a uniform location does not exist. If the uniform location does exist it returns an object. It should return an index number you can use in gl.uniformMatrix4fv.
Please fix so we can test in WebKit again.
This is a decision that has been made in the WebGL specification and discussed on the public mailing list.