ObservationId => base::UnguessableToken

It turns out to be much easier to use UnguessableToken with the
mojo binds for LearningTaskController, since multiple clients will
multiplex onto the same LearningTaskController.  This prevents
any aliasing between requests.

Change-Id: I07ec7e402d2300a17398f3331387c93d9daf81e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518808
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640141}
10 files changed