Generate index in parallel. This allows an index which used to take 3+ hours to be regenerated in ~11 minutes. BUG=None TEST=None Review URL: http://codereview.chromium.org/3034014 git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/gsd_generate_index@53006 0039d316-1c4b-4281-b951-d872f2087c98