beholder: use estimator from `tf.estimator` (#2004)
beholder: use estimator from `tf.estimator` (#2004)

Summary:
This reverts part of #1911. Tests seem to pass either way (on both
mainline nightly and the 2.0 preview), but the `tf.estimator` form
works better inside Google.

Test Plan:
Running `bazel test //tensorboard/...` passes on both Python 2 and
Python 3, with both tf-nightly-2.0-preview==2.0.0.dev20190312 and the
same version of tf-nightly.

Importing this tree into Google yields a working TensorBoard.

wchargin-branch: beholder-estimator-via-tf

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard