sync: speculative fix for bug 165561

The SyncManager will stop the scheduler as part of StopSyncingForShutdown, which
happens well before things like the invalidator are torn down / told to stop
interacting with the scheduler.  As such, we can wind up scheduling nudges on a lame
duck scheduler.  Some of the stacks associated with 165561 clearly show browser
shutdown happening on the UI thread and OnIncomingInvalidation happening on the
sync thread, which will crash on a second invocation as pending_nudge will be
set to a job that is dropped by ScheduleSyncSessionJob.

BUG=165561


Review URL: https://chromiumcodereview.appspot.com/11827013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175945 0039d316-1c4b-4281-b951-d872f2087c98

sync/engine/sync_scheduler_impl.cc

@@ -316,6 +316,7 @@ bool SyncSchedulerImpl::ScheduleConfiguration(
   DCHECK(IsConfigRelatedUpdateSourceValue(params.source));
   DCHECK_EQ(CONFIGURATION_MODE, mode_);
   DCHECK(!params.ready_task.is_null());
+  CHECK(started_) << "Scheduler must be running to configure.";
   SDVLOG(2) << "Reconfiguring syncer.";
 
   // Only one configuration is allowed at a time. Verify we're not waiting
@@ -594,6 +595,12 @@ void SyncSchedulerImpl::ScheduleNudgeImpl(
   DCHECK_EQ(MessageLoop::current(), sync_loop_);
   DCHECK(!invalidation_map.empty()) << "Nudge scheduled for no types!";
 
+  if (!started_) {
+    SDVLOG_LOC(nudge_location, 2)
+        << "Dropping nudge, scheduler is not running.";
+    return;
+  }
+
   SDVLOG_LOC(nudge_location, 2)
       << "In ScheduleNudgeImpl with delay "
       << delay.InMilliseconds() << " ms, "
@@ -992,6 +999,8 @@ void SyncSchedulerImpl::StopImpl(const base::Closure& callback) {
   weak_ptr_factory_.InvalidateWeakPtrs();
   wait_interval_.reset();
   poll_timer_.Stop();
+  unscheduled_nudge_storage_.reset();
+  pending_nudge_ = NULL;
   if (started_) {
     started_ = false;
   }