profile: fix loading progress behavior

[wchargin]

Summary:
The profile dashboard had an existing bug that was revealed by #1914:
the loading progress bar never actually finishes. Prior to #1914, you
could actually see the loading bar if you made your window narrow
enough:

This was because the prior version of the dashboard rendered the main
tool irrespective of whether the data had actually finished loading. As
of #1914, the “loading” and “active” states are mutually exclusive, so
instead of just having a zombie progress bar we actually block the view.

The cause of the bug is a promise that never resolves. (The executor
provided to the Promise constructor’s return value is ignored; only
the resolve and reject callbacks can fulfill a promise.)

This was not caught while testing #1914 because it cannot be reproduced
with the demo data present at that commit; with that more complete set
of data, there are other code paths that clobber the progress value to
100% (which is really a separate issue, but that’s not important right
now), such as _maybeUpdateData.

Test Plan:
Regenerate profile plugin demo data if you haven’t done so since #1942,
then launch TensorBoard:

$ rm -rf /tmp/profile_demo/
$ bazel run //tensorboard/plugins/profile:profile_demo
$ bazel run //tensorboard -- --logdir /tmp/profile_demo

The trace viewer should be displayed (prior to this commit, it wasn’t).

wchargin-branch: profile-broken-promises

[qiuminxu]

Thanks for the fix!