Ash: Fix a crash in BrowserShortcutLauncherItemController.

Actually mark the underlying pointer (not a copy) null.
Add a regression browsertest.

Bug: 937088
Test: Automated; fewer crash reports
Change-Id: I218f06ca875e0110490d304225478d32ed0d33a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660631
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669358}
3 files changed