[Sync] Tidy up SyncBackendHost interface.

Converted passphrase-related notifications to SyncFrontend methods.

Removed some unneeded parameters from SyncBackendHost constructor.

Moved some function definitions into .cc files.

BUG=None
TEST=unit_tests

Review URL: http://codereview.chromium.org/6380004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@71855 0039d316-1c4b-4281-b951-d872f2087c98