trunks: added ScopedKeyHandle class

BUG=chromium:415999
TEST=manual and unit

Change-Id: Ifdda27648e0c773f056b238050334e5d4fd58c76
Reviewed-on: https://chromium-review.googlesource.com/231391
Tested-by: Utkarsh Sanghi <usanghi@chromium.org>
Reviewed-by: Darren Krahn <dkrahn@chromium.org>
Commit-Queue: Utkarsh Sanghi <usanghi@chromium.org>
11 files changed