Update class comment of drive::JobScheduler.

In particular I wanted to clarify how the jobs are controlled
depending on network connections.

BUG=315000

Review URL: https://codereview.chromium.org/67813007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234483 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234483