Make sync's change processors sequence-affine.

components/sync/driver/shared_change_processor.h

Index: components/sync/driver/shared_change_processor.h
diff --git a/components/sync/driver/shared_change_processor.h b/components/sync/driver/shared_change_processor.h
index 3d4698f1441226e5043707484c87a53f2ce374e0..c1942375a34e4fdf29bfbd6c38b7fd8f4be3908d 100644
--- a/components/sync/driver/shared_change_processor.h
+++ b/components/sync/driver/shared_change_processor.h
@@ -12,6 +12,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "components/sync/driver/data_type_controller.h"
@@ -78,8 +79,8 @@ class SharedChangeProcessor
       std::unique_ptr<DataTypeErrorHandler> error_handler,
       const base::WeakPtr<SyncMergeResult>& merge_result);
 
-  // Disconnects from the generic change processor. May be called from any
-  // thread. After this, all attempts to interact with the change processor by
+  // Disconnects from the generic change processor. This method is thread-safe.
+  // After this, all attempts to interact with the change processor by
   // |local_service_| are dropped and return errors. The syncer will be safe to
   // shut down from the point of view of this datatype.
   // Note: Once disconnected, you cannot reconnect without creating a new
@@ -89,7 +90,7 @@ class SharedChangeProcessor
   virtual bool Disconnect();
 
   // GenericChangeProcessor stubs (with disconnect support).
-  // Should only be called on the same thread the datatype resides.
+  // Should only be called on the same sequence the datatype resides.
   virtual int GetSyncCount();
   virtual SyncError ProcessSyncChanges(
       const tracked_objects::Location& from_here,
@@ -143,11 +144,11 @@ class SharedChangeProcessor
   // destructed and/or disconnected on this loop, see ~SharedChangeProcessor.
   const scoped_refptr<const base::SingleThreadTaskRunner> frontend_task_runner_;
 
-  // The loop that all methods except the constructor, destructor, and
-  // Disconnect() should be called on.  Set in Connect().
-  scoped_refptr<base::SingleThreadTaskRunner> backend_task_runner_;
+  // The sequence that all methods except the constructor, destructor, and

[stanisc, 2016/12/21 18:48:09]

I personally find the term sequence confusing in this context - "sequence that a
method should be called on". This term is way too general and usually used for
something completely different (e.g. data sequence). 

Thread might not be the right term anymore but at least it was less ambiguous
and easier to understand.

[gab, 2016/12/21 18:52:17]

"execution sequence"?

[stanisc, 2016/12/21 18:57:27]

execution sequence is better. Or task sequence.
Or something new that is shorter and less general.

[fdoray, 2016/12/21 19:12:51]

Done.

+  // Disconnect() should be called on. Set in Connect().
+  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
 
-  // Used only on |backend_loop_|.
+  // Used only on |backend_task_runner_|.
   GenericChangeProcessor* generic_change_processor_;
 
   std::unique_ptr<DataTypeErrorHandler> error_handler_;