Revert of Let SyncBackupManager keep backup data in memory until shutdown. Only persist

Revert of Let SyncBackupManager keep backup data in memory until shutdown. Only persist (patchset #3 of https://codereview.chromium.org/455023003/)

Reason for revert:
sync_unit_tests broke on Linux dbg bot.

http://build.chromium.org/p/chromium.linux/buildstatus?builder=Linux%20Tests%20%28dbg%29%282%29&number=48388

------------------------------------------------------------

[ RUN      ] SyncManagerTestWithMockScheduler.ReConfiguration

GMOCK WARNING:
Uninteresting mock function call - returning directly.
Function call: Start(0)
NOTE: You can safely ignore the above warning unless this call should not happen.  Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call.  See http://code.google.com/p/googlemock/wiki/CookBook#Knowing_When_to_Expect for details.
Stack trace:
../../sync/internal_api/sync_manager_impl_unittest.cc:838: Failure
Value of: storage_used_
Actual: 2880154539
Expected: InternalComponentsFactory::STORAGE_ON_DISK
Which is: 1
[  FAILED  ] SyncManagerTestWithMockScheduler.ReConfiguration (34 ms)
[828/829] SyncManagerTestWithMockScheduler.ReConfiguration (34 ms)
[0814/160010:ERROR:kill_posix.cc(191)] Unable to terminate process group 6975: No such process
[ RUN      ] SyncManagerTestWithMockScheduler.BasicConfiguration

GMOCK WARNING:
Uninteresting mock function call - returning directly.
Function call: Start(0)
NOTE: You can safely ignore the above warning unless this call should not happen.  Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call.  See http://code.google.com/p/googlemock/wiki/CookBook#Knowing_When_to_Expect for details.
Stack trace:
../../sync/internal_api/sync_manager_impl_unittest.cc:838: Failure
Value of: storage_used_
Actual: 2880154539
Expected: InternalComponentsFactory::STORAGE_ON_DISK
Which is: 1
[  FAILED  ] SyncManagerTestWithMockScheduler.BasicConfiguration (29 ms)
[829/829] SyncManagerTestWithMockScheduler.BasicConfiguration (29 ms)

Original issue's description:
> Let SyncBackupManager keep backup data in memory until shutdown. Only persist
> backup data to disk if shutdown is due to switching to sync.
> 
> BUG=362679
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289709

TBR=zea@chromium.org,haitaol@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=362679
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289718

[miu, 2014-08-14 23:05:24]

Created Revert of Let SyncBackupManager keep backup data in memory until
shutdown. Only persist

[commit-bot: I haz the power, 2014-08-14 23:07:52]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/477813002/1

[commit-bot: I haz the power, 2014-08-14 23:11:42]

Committed patchset #1 (1) as 289718