Call measure and baseline fns within YGNode

Summary:
@public

Stricter encapsulation of baseline and measure callbacks withing `YGNode`.

Instead of invoking these callbacks directly (`node->getBaseline()(...)`), they are invoked via methods on `YGNode` (`node->baseline(...)`).

This change will allow us to add the concept of a *Layout Context,* where measure and baseline functions will be able to receive an additional `void *` argument if configured accordingly. This API will be used internally for Yoga’s JNI bindings, to avoid storing a weak JNI reference for each node, and avoid reference table overflows.

Changed API:

- `YGNodeGetMeasureFunc()` -> `YGNodeHasMeasureFunc()`
- `YGNodeGetBaselineFunc()` -> `YGNodeHasBaselineFunc()`
- `YGNode::getMeasure()` -> `YGNode::hasMeasureFunc()` + `YGNode::measure()`
- `YGNpde::getBaseline()` -> `YGNode::hasBaselineFunc()` + `YGNode::baseline()`

Reviewed By: SidharthGuglani

Differential Revision: D14099550

fbshipit-source-id: 2653ab36acc252a9747986bc88d21dac22d8c91b

yoga/YGNode.cpp

@@ -270,8 +270,8 @@ YGNode& YGNode::operator=(const YGNode& node) {
   print_ = node.getPrintFunc();
   hasNewLayout_ = node.getHasNewLayout();
   nodeType_ = node.getNodeType();
-  measure_ = node.getMeasure();
-  baseline_ = node.getBaseline();
+  measure_ = node.measure_;
+  baseline_ = node.baseline_;
   dirtied_ = node.getDirtied();
   style_ = node.style_;
   layout_ = node.layout_;