web.bzl: use Skylint-recommended re-export syntax (#1669)
web.bzl: use Skylint-recommended re-export syntax (#1669)

Summary:
Per the [“Skylint: Re-exporting `load()`ed names” docs on the Bazel
website][1].

[1]: https://docs.bazel.build/versions/master/skylark/skylint.html#re-exporting-loaded-names

Test Plan:
That existing tests build and pass is sufficient.

wchargin-branch: re-export-syntax-fixup
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard