ExecutionQueue takes care of reordering execution depending on each Dependency
requirements. There is no need to sort anymore.

ExecutionQueue also manages the Progress instance and is multithread safe, in preparation to parallel checkout.

Review URL: http://codereview.chromium.org/3112002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@55882 0039d316-1c4b-4281-b951-d872f2087c98