Call batch/incremental fetch with more strict conditions

Call batch/incremental fetch with more strict conditions

- Deprecate is_fetching_changes_ flag
- Add a new flag: may_have_unfetched_changes_ which is turned on
  at the beginning and when an event is fired
- Call StartBatchSyncForOrigin() and FetchChangesForIncrementalSync()
  only from MaybeStartFetchTask() with more strict conditions.

Partialy integrating the change proposal made by tzik:
https://codereview.chromium.org/12221088/

* It ALWAYS sets may_have_unfetched_changes_ true and call MaybeStartFetchTask()
in XMPP/Timer/Auth/Network event handlers, so it has to be able to kick
StartBatchSyncForOrigin() or FetchChangesForIncrementalSync() whenever an
event happens even if the service state is in TEMPORARY_UNAVAILABLE.

* It ALWAYS sets may_have_unfetched_changes_ false in StartBatchSyncForOrigin()
and FetchChangesForIncrementalSync(), so that it won't get stuck in an infinite
loop calling one of the methods again and again when the service state is
TEMPORARY_UNAVAILABLE (e.g. network is offline).

* It does NOT queue fetch tasks to the pending_tasks_ when another task is
running, so the service has not to be overloaded by unifinished pending fetch
tasks.

* NotifyTaskDone() ALWAYS calls SchedulePolling() (which starts the timer
if it hasn't been started) so it should not miss the tminigs where one of
fetch-change methods have to be called.

== State transition map:

{ OK, !batch.empty(), _ }
  -> NotifyTaskDone() calls StartBatchSyncForOrigin() until batch.empty() becomes true

{ TEMPORARY_UNAVAILABLE, !batch.empty(), _ }
  -> NotifyTaskDone() calls nothing
  -> XMPP/Timer/Auth/Network event handler calls StartBatchSyncForOrigin() if no task is running, otherwise let NotifyTaskDone() call it at most once

{ !DISABLED, batch.empty(), !incremental.empty() }
  -> NotifyTaskDone() calls nothing
  -> XMPP/Timer/Auth/Network event handler calls FetchChangesForIncrementalSync() if no task is running, otherwise let NotifyTaskDone() call it at most once

BUG=175047
TEST=manual

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=181851

[kinuko, 2013-02-11 13:17:29]

wdyt

[tzik, 2013-02-12 03:36:38]

lgtm

[kinuko, 2013-02-12 04:48:06]

Thanks, ok I'm submitting this now...