Fix landed at r542604 and first shipped in 67.0.3369.0.

It's actually worse than the summary says. Prior to the fix:
- per-machine Desktop launches appeared in the LM_SHORTCUT_UNKNOWN bucket
- per-user Start Menu launches appeared in the LM_SHORTCUT_DESKTOP bucket
- per-machine Start Menu launches appeared in the LM_SHORTCUT_UNKNOWN bucket

So we expect the metric to change with the fix like so:
- some portion of per-user LM_SHORTCUT_DESKTOP will move to LM_SHORTCUT_START_MENU
- some portion (most, i hope) of per-machine LM_SHORTCUT_UNKNOWN will move to LM_SHORTCUT_DESKTOP and LM_SHORTCUT_START_MENU

I think we should merge this fix to M66 once we validate data collected from dev channel (where we have both per-user and per-machine installs).

Metrics look good on canary and dev channels. I see no crashes related to the modified code. I'd like to merge r542604 to M66 so that we can start getting meaningful data from stable channel sooner. Thanks for considering.

This bug requires manual review: M66 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), josafat@(ChromeOS), abdulsyed@(Desktop)

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Approving merge to M66. Branch:3359

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f9cd05216779df16b4960343313204a81c7bb36e

commit f9cd05216779df16b4960343313204a81c7bb36e
Author: Greg Thompson <grt@chromium.org>
Date: Tue Mar 20 10:53:06 2018

Add LM_SHORTCUT_START_MENU (per-user and per-machine) to Launch.Modes histogram.

Also:
- Update the code to use PathService to get various locations.
- Check both per-user and per-machine locations for Desktop shortcuts.
- Remove stale comments and replace a stale constant with its documented
  name.
- Perform expensive checks outside of the critical path to startup.

BUG= 820577 
R=​tmartino@chromium.org
TBR=grt@chromium.org

Change-Id: Ib8e4cdb91def96660095fbfbb18ae59e77d8e8da
Reviewed-on: https://chromium-review.googlesource.com/955655
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#542604}(cherry picked from commit e4ad6dee3c362cbf84309c9bf78c6f8fd1f8e611)
Reviewed-on: https://chromium-review.googlesource.com/970581
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/branch-heads/3359@{#339}
Cr-Branched-From: 66afc5e5d10127546cc4b98b9117aff588b5e66b-refs/heads/master@{#540276}
[modify] https://crrev.com/f9cd05216779df16b4960343313204a81c7bb36e/chrome/browser/ui/startup/startup_browser_creator_impl.cc
[modify] https://crrev.com/f9cd05216779df16b4960343313204a81c7bb36e/tools/metrics/histograms/enums.xml