support flexible top container

Summary:
The main container that gets called `applyLayoutPreservingOrigin:` is using its size as a fixed bounding box.
In some cases it's preferrable to let it accomodate its contents.

This diffs extends `applyLayoutPreservingOrigin:` by adding an additional parameter which can be used to sepecify whether width and/or height are fixed or flexible.

Feel free to suggest better names than `YGDimensionFlexibility` & co.
Let me know if you prefet to kill the API without flexiblity specifier - I'll codemod everything then.

Reviewed By: dshahidehpour

Differential Revision: D4929702

fbshipit-source-id: f128f244140b4a54d8ce3b3f4edddbb9756f8fdf

YogaKit/Source/YGLayout.m

@@ -235,6 +235,20 @@ YG_PROPERTY(CGFloat, aspectRatio, AspectRatio)
   YGApplyLayoutToViewHierarchy(self.view, preserveOrigin);
 }
 
+- (void)applyLayoutPreservingOrigin:(BOOL)preserveOrigin dimensionFlexibility:(YGDimensionFlexibility)dimensionFlexibility
+{
+  CGSize size = self.view.bounds.size;
+  if (dimensionFlexibility & YGDimensionFlexibilityFlexibleWidth) {
+    size.width = YGUndefined;
+  }
+  if (dimensionFlexibility & YGDimensionFlexibilityFlexibleHeigth) {
+    size.height = YGUndefined;
+  }
+  [self calculateLayoutWithSize:size];
+  YGApplyLayoutToViewHierarchy(self.view, NO);
+}
+
+
 - (CGSize)intrinsicSize
 {
   const CGSize constrainedSize = {