Files app: Add metrics to record root type of chosen files in OPEN dialog.

When Files app is opened as an file picker to select files to open, we'd
like to know the location of the chosen files.
Locations can be "Downloads", "Google Drive", "Recent", "Removable", etc..

Bug: 760407
Test: Tested using chrome://histograms.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3a24ca4c4c74b68aa2a88b31f2906f3abb6e678c
Reviewed-on: https://chromium-review.googlesource.com/711817
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508618}
6 files changed