Add YGLayoutGetMargin

Summary:
Fix #326. I'll open another PR once this one gets accepted to add support for `YGLayoutGetBorder` 👌
Closes https://github.com/facebook/yoga/pull/335

Reviewed By: gkassabli

Differential Revision: D4409399

Pulled By: emilsjolander

fbshipit-source-id: 8153f6701cab60b55a485f6d2e0b9f7767481090

tests/YGComputedMarginTest.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_margin) {
+  const YGNodeRef root = YGNodeNew();
+  YGNodeStyleSetWidth(root, 100);
+  YGNodeStyleSetHeight(root, 100);
+  YGNodeStyleSetMarginPercent(root, YGEdgeStart, 10);
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionLTR);
+
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetMargin(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetMargin(root, YGEdgeRight));
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionRTL);
+
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetMargin(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetMargin(root, YGEdgeRight));
+
+  YGNodeFreeRecursive(root);
+}