Add checkpoint/rollback to ReactNativeReconcileTransaction (#7619)

(cherry picked from commit 51f04fd)

src/renderers/native/ReactNativeReconcileTransaction.js

@@ -97,6 +97,19 @@ var Mixin = {
     return ReactUpdateQueue;
   },
 
+  /**
+   * Save current transaction state -- if the return value from this method is
+   * passed to `rollback`, the transaction will be reset to that state.
+   */
+  checkpoint: function() {
+    // reactMountReady is the our only stateful wrapper
+    return this.reactMountReady.checkpoint();
+  },
+
+  rollback: function(checkpoint) {
+    this.reactMountReady.rollback(checkpoint);
+  },
+
   /**
    * `PooledClass` looks for this, and will invoke this before allowing this
    * instance to be reused.