LiveDevMultiBrowser should properly search parent directories for an index.html

Presently, LiveDevMultiBrowser gets stuck in an infinite loop trying to step into parent directories to find an initial index.html if none is present in the current directory. Without the fix, the included test will loop forever.
Of course, eventually we should clean this up even further since almost the entire body of `_getInitialDocFromCurrent()` is duplicate code (and I'm guessing a bunch of the other code in this file too).  It's unclear how long LiveDevelopment.js needs to exist independently, though -- the biggest open question to me is whether we'd need to keep the old implementation around for a long time to support the custom-server use case.  A good discussion to start 2015 with, maybe :-)