Add a devserver call that can list staged contents of a build.

This will help with knowing why an update failed.

TEST=Executed the call on a local devserver. Added a unittest.
BUG=chromium:348097, chromium:349044, chromium:349047

Change-Id: I9c1a1a0ba9212852ad243eec728a010b5ed0b0fc
Tested-by: Prashanth B <>
Reviewed-by: Dan Shi <>
Reviewed-by: Chris Sosa <>
Commit-Queue: Prashanth B <>
3 files changed