Do not assume that all presubmit directories still exist.
Do not assume that all presubmit directories still exist.

In the case where a CL deletes the last file in the directory,
it is listed in the presubmit files list, but the directory to
which it refers will not exist, leading to os.listdir failing.

BUG=638343

Review-Url: https://codereview.chromium.org/2250353003

primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools