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

java/com/facebook/yoga/YogaNode.java

@@ -70,6 +70,14 @@ public class YogaNode implements YogaNodeAPI<YogaNode> {
   @DoNotStrip
   private float mLeft = YogaConstants.UNDEFINED;
   @DoNotStrip
+  private float mMarginLeft = 0;
+  @DoNotStrip
+  private float mMarginTop = 0;
+  @DoNotStrip
+  private float mMarginRight = 0;
+  @DoNotStrip
+  private float mMarginBottom = 0;
+  @DoNotStrip
   private float mPaddingLeft = 0;
   @DoNotStrip
   private float mPaddingTop = 0;
@@ -573,6 +581,26 @@ public class YogaNode implements YogaNodeAPI<YogaNode> {
     return mHeight;
   }
 
+  @Override
+  public float getLayoutMargin(YogaEdge edge) {
+    switch (edge) {
+      case LEFT:
+        return mMarginLeft;
+      case TOP:
+        return mMarginTop;
+      case RIGHT:
+        return mMarginRight;
+      case BOTTOM:
+        return mMarginBottom;
+      case START:
+        return getLayoutDirection() == YogaDirection.RTL ? mMarginRight : mMarginLeft;
+      case END:
+        return getLayoutDirection() == YogaDirection.RTL ? mMarginLeft : mMarginRight;
+      default:
+        throw new IllegalArgumentException("Cannot get layout margins of multi-edge shorthands");
+    }
+  }
+
   @Override
   public float getLayoutPadding(YogaEdge edge) {
     switch (edge) {

java/com/facebook/yoga/YogaNodeAPI.java

@@ -82,6 +82,7 @@ public interface YogaNodeAPI<YogaNodeType extends YogaNodeAPI> {
   float getLayoutY();
   float getLayoutWidth();
   float getLayoutHeight();
+  float getLayoutMargin(YogaEdge edge);
   float getLayoutPadding(YogaEdge edge);
   YogaDirection getLayoutDirection();
   YogaOverflow getOverflow();