The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/66d336cc90b6e32986ee06247fdf07023cfda87f

commit 66d336cc90b6e32986ee06247fdf07023cfda87f
Author: Joel Hockey <joelhockey@chromium.org>
Date: Mon Jun 04 12:42:52 2018

FilesApp add UMA for time to start crostini container

Bug:  845394 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5f4360e7b2eb049576078fdefec4f38039a36b9c
Reviewed-on: https://chromium-review.googlesource.com/1072219
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564068}
[modify] https://crrev.com/66d336cc90b6e32986ee06247fdf07023cfda87f/tools/metrics/histograms/histograms.xml
[modify] https://crrev.com/66d336cc90b6e32986ee06247fdf07023cfda87f/ui/file_manager/file_manager/foreground/js/directory_contents.js
[modify] https://crrev.com/66d336cc90b6e32986ee06247fdf07023cfda87f/ui/file_manager/file_manager/test/js/chrome_api_test_impl.js

Still to be done:
* Check with weifangsun/tbuckley for any further UMAs.
 - so far I've added UMA for start/mount time which only records successful cases
* Add tests