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platform_util contains utilities for showing a file in a file explorer
and opening a URL with an external protocol handler. These will need
to be plumbed from lacros to ash via crosapi.

Introduce a stub file for lacros.  Rename the existing _chromeos file
to _ash to match our naming scheme.

No functional changes.

Bug: 1139128, 1140585
Change-Id: I51d50f3687010f8397c8ea584cce21caccbe6982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488084
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819892}