Plug resource leaks in ShortcutsBackendTest.

Explicitly shut down the ShortcutsBackend and HistoryService in TearDown
before cleaning up the ScopedTempDir.

BUG=546640

Change-Id: I961dc49376116550e63a9c353d3d15a436731e93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943130
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Kevin Bailey <krb@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720359}
1 file changed