[Sync] Add plumbing of context from client to server

sync/internal_api/public/write_transaction.h

Index: sync/internal_api/public/write_transaction.h
diff --git a/sync/internal_api/public/write_transaction.h b/sync/internal_api/public/write_transaction.h
index 9008b4f2b8a35ef72dc5f5f56da0bdad9564a1ba..8e797beae21c7fd05c0da23c18d34bf37c18e9d7 100644
--- a/sync/internal_api/public/write_transaction.h
+++ b/sync/internal_api/public/write_transaction.h
@@ -43,6 +43,9 @@ class SYNC_EXPORT WriteTransaction : public BaseTransaction {
   virtual syncable::BaseTransaction* GetWrappedTrans() const OVERRIDE;
   syncable::WriteTransaction* GetWrappedWriteTrans() { return transaction_; }
 
+  // Set's a |type|'s local context. Does not affect any individual entities.
+  void SetDataTypeContext(ModelType type, const std::string& context);
+
  protected:
   WriteTransaction() {}