cros: Fix session restore on connection change.

cros: Fix session restore on connection change.

- ContinueSessionRestore() should ensure SESSION_RESTORE_PREPARING state.
  Otherwise, /MergeSession is not called after /ListAccount because it would
  not be considered as is_pre_merge;
- Cover SESSION_RESTORE_PREPARING state in connection change event;

BUG=487476
Committed: https://crrev.com/8f05571bc82372de9880fca638fc9ff00024238b
Cr-Commit-Position: refs/heads/master@{#329606}

[xiyuan, 2015-05-13 02:29:00]

Patchset #2 (id:20001) has been deleted

[xiyuan, 2015-05-13 02:38:39]

xiyuan@chromium.org changed reviewers:
+ nkostylev@chromium.org

[Nikita (slow), 2015-05-13 08:27:20]

The CQ bit was checked by nkostylev@chromium.org

[Nikita (slow), 2015-05-13 08:27:20]

lgtm

[commit-bot: I haz the power, 2015-05-13 08:27:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1137253003/40001

[commit-bot: I haz the power, 2015-05-13 08:30:42]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-05-13 08:31:40]

Patchset 2 (id:??) landed as
https://crrev.com/8f05571bc82372de9880fca638fc9ff00024238b
Cr-Commit-Position: refs/heads/master@{#329606}