Associate data types on different browser threads in parallel.

This CL changes ModelAssociationManager to associate multiple data types on
different threads in parallel. This can speed up first-sync by 20% to 50% on various platforms. 

BUG=325794

Review URL: https://codereview.chromium.org/94093006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240415 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240415