Rename variables in update_reference_builds.py, and add comments.

After trying to run update_reference_builds, I found that when trying to get an official Chrome build, what I had to give was a Chrome version number (like 34.0.1797.1).

Although "version", "revision" and "build number" all have similar meanings and different meanings in different contexts, my understanding is that:
  Chrome version number = 4 numbers separated by dots, as described at http://www.chromium.org/developers/version-numbers.
  Chromium SVN revision number: A number like 251904.
  Build number: May refer to the third number in the Chrome version number (e.g. 1797).

The purpose of this CL is to change variable names so that they reflect these definitions.

BUG=

Review URL: https://codereview.chromium.org/173203003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254694 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254694