Skip forcing layout when UICollectionView is invisible #trivial (#1268)

Summary:
Issue fixed: # https://github.com/Instagram/IGListKit/issues/1267

- [ ] All tests pass. Demo project builds and runs.
- [ ] I added tests, an experiment, or detailed why my change isn't tested.
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [ ] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1268

Reviewed By: maxoll

Differential Revision: D12839019

Pulled By: rnystrom

fbshipit-source-id: c695d66234d7d1969f9f17f55cbe9d7967970adc

Source/Common/IGListExperiments.h

@@ -24,6 +24,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentDeferredToObjectCreation = 1 << 6,
     /// Test getting collection view at update time.
     IGListExperimentGetCollectionViewAtUpdate = 1 << 7,
+    /// Test skipping layout when UICollectionView isn't visible
+    IGListExperimentSkipLayout = 1 << 8,
 };
 
 /**

Source/IGListAdapterUpdater.m

@@ -176,7 +176,10 @@
         [self _cleanStateAfterUpdates];
         [self _performBatchUpdatesItemBlockApplied];
         [collectionView reloadData];
-        [collectionView layoutIfNeeded];
+        if (!IGListExperimentEnabled(self.experiments, IGListExperimentSkipLayout)
+            || collectionView.window != nil) {
+            [collectionView layoutIfNeeded];
+        }
         executeCompletionBlocks(YES);
     };