Simplify ExecutionQueue.run() by moving code into utility functions.

Reduce the amount of code in WorkerThread.run() to improve reliability.

Don't trap exceptions in single-threaded usage!

BUG=54084
TEST=none

Review URL: http://codereview.chromium.org/3336015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58974 0039d316-1c4b-4281-b951-d872f2087c98