code: public <T, U> ExecutionChain runOnGLThread(Task<T, U> task) {
          runOnThread(ExecutionChain.Context.Type.GL, task);
          return this;
      }
docs: Add a {@link Task} to be run on the
  {@link GVRContext#runOnGlThread(Runnable) OpenGL thread}. It will be run
  after all Tasks added prior to this call.
  
  @param task
  {@code Task} to run
  @return Reference to the {@code ExecutionChain}.
  @throws IllegalStateException
  if the chain of execution has already been {@link #execute()
  started}.