Bug 966104 - Part 2: Add FirefoxAccounts.requestSync.
Bug 966104 - Part 3: Test FirefoxAccounts.extrasForSync.
Bug 966104 - Part 4: Force a sync after updating credentials.
Bug 966104 - Part 5: Add and use FxAccountSyncStatusHelper.
Bug 966104 - Part 6: Log sync hints in FxAccountSyncAdapter.