Add UMA for catalog loading:

1. Add UMA for which path caused the load, background, EoS, or NTP.
2. Add UMA for the outcome of the load, new catalog, unchanged, or failure.

Also add unit tests for the C++ side histograms.

Bug: 910429
Change-Id: I56e06b1dcc5b7f977fd3646e199767c1194c4c02
Reviewed-on: https://chromium-review.googlesource.com/c/1357211
Commit-Queue: Peter Williamson <petewil@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616164}
11 files changed