Add upgrade hooks to support removing chrome projects from manifest.

See the bug for details.

BUG=chromium-os:32963.
TEST=Locally, remote trybots.

Change-Id: I33f5c42b36f3e06139036c299c2fc2c2ff026411
Reviewed-on: https://gerrit.chromium.org/gerrit/28543
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
2 files changed