sync: Support non-blocking initial sync in proto

sync/engine/sync_scheduler_impl.cc

Index: sync/engine/sync_scheduler_impl.cc
diff --git a/sync/engine/sync_scheduler_impl.cc b/sync/engine/sync_scheduler_impl.cc
index 4d38c21f87a8f68ba5cb34112ae4bf6521a4b461..974ed27e4ee9a9e45f7228e6b31afb6ce4b374da 100644
--- a/sync/engine/sync_scheduler_impl.cc
+++ b/sync/engine/sync_scheduler_impl.cc
@@ -398,6 +398,15 @@ void SyncSchedulerImpl::ScheduleInvalidationNudge(
   ScheduleNudgeImpl(desired_delay, nudge_location);
 }
 
+void SyncSchedulerImpl::ScheduleInitialSyncNudge(syncer::ModelType model_type) {

[stanisc, 2014/07/17 18:46:10]

This is called ScheduleInitialSyncNudge. Does the "initial" part imply that this
should be called only once per model_type? Would calling this multiple times be
a problem?

[rlarocque, 2014/07/17 18:58:06]

It should be very uncommon that it gets called more than once per ModelType, but
not impossible.  

We should to call it every time we need to ask the server to re-sync our type
when we're starting from nothing.  This will happen when the user first signs
in.  It can also happen if we've deleted our local sync data because a user
disabled then re-enabled sync for this type.  Or it could be that the server
requested a "migration", which is to say that it asked the client to clear its
sync data and start syncing from scratch.

The effects of calling it unnecessarily won't be too severe.  It would cause the
client to perform a GetUpdate request.  The server would (1) be forced to read
from its storage, since it is not allowed to "optimize" requests that carry this
flag, and (2) log this as an initial sync request, which might have an impact on
our graphs or metrics.

We certainly don't want to call it for no reason, but it's reasonably safe in
that there would be no data corruption if we did.

+  DCHECK(CalledOnValidThread());
+
+  SDVLOG(2) << "Scheduling non-blocking initial sync for "
+            << ModelTypeToString(model_type);
+  nudge_tracker_.RecordInitialSyncRequired(model_type);
+  ScheduleNudgeImpl(TimeDelta::FromSeconds(0), FROM_HERE);
+}
+
 // TODO(zea): Consider adding separate throttling/backoff for datatype
 // refresh requests.
 void SyncSchedulerImpl::ScheduleNudgeImpl(