Make Source.getDirectory synchronous.

It never really made sense for this to be asynchronous; most sources
didn't even do any IO to determine the directory. The only exception was
the git source, which would actually *clone a repository* if one didn't
exist yet. This heavyweight behavior wasn't a good fit for what looks
like a lightweight method anyway.

Making this synchronous has a ripple effect of making a bunch of other
methods synchronous, too. Hooray for synchrony!

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1276673006 .