I believe you need to clear uniform values here. It's a bit of a dance, but:
```
var uniformCount = gl.getProgramParameter(program, goog.webgl.ACTIVE_UNIFORMS);
for (var n = 0; n < uniformCount; n++) {
  var uniformInfo = gl.getActiveUniform(program, n);
  var uniformLocation = gl.getUniformLocation(program, uniformInfo.name);
  switch (uniformInfo.type) {
    ...
    case goog.webgl.FLOAT:
      switch (uniformInfo.size) {
        case 1:
          gl.uniform1f(uniformLocation, 0);
          break;
        ...
      }
      break;
  }
}
```

Should be straight forward.
You'll need this form all the uniform types (int/float) and sizes (1-4) as well as matrices (2-4). Have a `resetProgramUniforms_` or something.