Bug 1077529 - Update Trusted Hosted Apps r=fabrice

Verify manifest of trusted hosted app on update instead
of just overwriting it.
---
 dom/apps/TrustedHostedAppsUtils.jsm |  8 ++++----
 dom/apps/Webapps.jsm                | 26 +++++++++++++++++++++-----
 2 files changed, 25 insertions(+), 9 deletions(-)