[SyncFS] Make task invocation in SyncTaskRunner async

[SyncFS] Make task invocation in SyncTaskRunner async

SyncTaskRunner's task invocation tend to grow call stack deeper.
This CL clears the call stack per task to avoid stack overflow.

BUG=344769
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278362

[tzik, 2014-06-18 06:47:56]

PTL

[peria, 2014-06-18 06:58:00]

https://codereview.chromium.org/334053008/diff/1/chrome/browser/sync_file_sys...
File chrome/browser/sync_file_system/drive_backend/sync_task_manager_unittest.cc
(right):

https://codereview.chromium.org/334053008/diff/1/chrome/browser/sync_file_sys...
chrome/browser/sync_file_system/drive_backend/sync_task_manager_unittest.cc:649:
// task1 blocks task2. task2's update should be pending until task1 update.
The following code expects different behaviors from this comment.
Which one is correct?

[tzik, 2014-06-19 04:04:57]

https://codereview.chromium.org/334053008/diff/1/chrome/browser/sync_file_sys...
File chrome/browser/sync_file_system/drive_backend/sync_task_manager_unittest.cc
(right):

https://codereview.chromium.org/334053008/diff/1/chrome/browser/sync_file_sys...
chrome/browser/sync_file_system/drive_backend/sync_task_manager_unittest.cc:649:
// task1 blocks task2. task2's update should be pending until task1 update.
On 2014/06/18 06:58:00, peria wrote:
> The following code expects different behaviors from this comment.
> Which one is correct?

Updated the comment: "be pending" -> "not complete".
task2's update begins at this point, but does not complete until task1's
completion.

[peria, 2014-06-19 04:11:05]

lgtm

[tzik, 2014-06-19 07:06:34]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2014-06-19 07:08:49]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tzik@chromium.org/334053008/20001

[commit-bot: I haz the power, 2014-06-19 14:32:44]

Change committed as 278362