[Sync] Add sync api support for context changes

sync/api/sync_change.h

Index: sync/api/sync_change.h
diff --git a/sync/api/sync_change.h b/sync/api/sync_change.h
index 8751fa1ea1b68d58aba469230d320254b63c7c05..6c9e95dd16275a91a34b6da5e0c95bac9a515d27 100644
--- a/sync/api/sync_change.h
+++ b/sync/api/sync_change.h
@@ -15,8 +15,9 @@
 
 namespace syncer {
 
-// A SyncChange object reflects a change to a piece of synced data. The change
-// can be either a delete, add, or an update. All data relevant to the change
+// A SyncChange object reflects a change to a piece of synced data. If the
+// change is to a a sync entity, the change can be either a delete, add, or an

[maniscalco, 2014/03/31 23:59:16]

nit: s/to a a/to a/

[maniscalco, 2014/03/31 23:59:16]

nit: s/either //

[Nicolas Zea, 2014/04/02 18:34:36]

Done.

[Nicolas Zea, 2014/04/02 18:34:36]

Done.

+// update. Context changes are always updates. All data relevant to the change
 // is encapsulated within the SyncChange, which, once created, is immutable.
 // Note: it is safe and cheap to pass these by value or make copies, as they do
 // not create deep copies of their internal data.
@@ -27,6 +28,7 @@ class SYNC_EXPORT SyncChange {
     ACTION_ADD,
     ACTION_UPDATE,
     ACTION_DELETE,
+    CONTEXT_UPDATE,

[maniscalco, 2014/03/31 23:59:16]

It would be helpful to have a comment somewhere (here or elsewhere) that
explains what a context update is.

[maniscalco, 2014/03/31 23:59:16]

I would have expected this to be called ACTION_CONTEXT_UPDATE.  Is there a
reason to avoid calling it that?  I guess I'm still fuzzy on how to think about
a context update relative to other SyncChanges.

[Nicolas Zea, 2014/04/02 18:34:36]

I was kind of thinking to leave the ACTION_ prefixes for those changes to actual
sync entities. WDYT?

[Nicolas Zea, 2014/04/02 18:34:36]

Done.

[maniscalco, 2014/04/02 21:13:09]

I'm OK with that distinction.  My thinking here was that it's handy to have a
way to easily search for all uses of the SyncChangeType enum (e.g. grep
SyncChange::ACTION_).  I guess having CONTEXT_UPDATE doesn't change the
searchability all that much because we can search for SyncChange::ACTION_ and
SyncChange::CONTEXT_.

   };
 
   // Default constructor creates an invalid change.