[SyncFS] Support late authentication

[SyncFS] Support late authentication

- Make observers to external services sticky over re-initialization.
- Leave SyncWorker NULL when non sign-in state.
- Observe sign-in event and trigger initialization on it.

BUG=374101, 380615
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276591

[tzik, 2014-06-11 10:13:12]

PTL

[peria, 2014-06-11 10:51:55]

lgtm

[tzik, 2014-06-11 11:13:55]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2014-06-11 11:16:51]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tzik@chromium.org/324343002/20003

[commit-bot: I haz the power, 2014-06-11 20:39:29]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[commit-bot: I haz the power, 2014-06-11 21:04:21]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-11 21:04:22]

Try jobs failed on following builders:
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[tzik, 2014-06-12 01:51:36]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2014-06-12 01:54:34]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tzik@chromium.org/324343002/20003

[commit-bot: I haz the power, 2014-06-12 02:54:49]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[tzik, 2014-06-12 03:51:20]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2014-06-12 03:53:42]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tzik@chromium.org/324343002/50001

[commit-bot: I haz the power, 2014-06-12 08:59:27]

Change committed as 276591