Add api for retrieving computed padding

Summary: Add API for retrieving the computed final padding of a node. Many frameworks such as React Native retrieve padding via `YGNodeStyleGetPadding` but given that we now support percentage values this is not correct anymore.

Differential Revision: D4376572

fbshipit-source-id: 3ffb66e77090fc1257511bec5c933f9b0c304b9f

tests/YGComputedPaddingTest.cpp

@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#include <yoga/Yoga.h>
+#include <gtest/gtest.h>
+
+TEST(YogaTest, computed_layout_padding) {
+  const YGNodeRef root = YGNodeNew();
+  YGNodeStyleSetWidth(root, 100);
+  YGNodeStyleSetHeight(root, 100);
+  YGNodeStyleSetPaddingPercent(root, YGEdgeStart, 10);
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionLTR);
+
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetPadding(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetPadding(root, YGEdgeRight));
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionRTL);
+
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetPadding(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetPadding(root, YGEdgeRight));
+
+  YGNodeFreeRecursive(root);
+}