ResourceLoaderStartUpModule: Improve comment about modifiedTime hack

* Re-ordering code to bring the modifiedTime hack and the
  loop it serves closer together.
* Separating the fact that it needs a value and the mtime of
  startup.js since that is just one of the three factors we use.
  This way it's clearer that the startup.js mtime is not just a
  bogus value, but not more or less important than wgCacheEpoch
  and modules mtime either.
* Remove duplicate '/* Methods */' comment, we already have this
  marker a few methods higher up.

Change-Id: Id3a07f02566c0f04b612b81f8353f70fa4ab3977
Merge "ResourceLoaderStartUpModule: Improve comment about modifiedTime hack"