Dont measure single flex grow+shrink child

Summary:
If there is a single child which is flex grow and flex shrink then instead of measuring and then shrinking we can just set the flex basis to zero as we know the final result will be that the child take up all remaining space.

This is a re-land of D4147298. I have updated the diff to check explicitly for exact measure mode to also handle at_most case correctly.

Reviewed By: gkassabli

Differential Revision: D4153133

fbshipit-source-id: 2333150a83857cc30078cc8d52761cbd00652830

csharp/tests/Facebook.CSSLayout/CSSLayoutFlexTest.cs

@@ -41,6 +41,12 @@
   <div style="height: 10px; flex-grow:1;"></div>
   <div style="height: 10px; flex-grow:1;"></div>
 </div>
+
+<div id="flex_grow_shrink_at_most" style="height: 100px; width: 100px;">
+  <div>
+    <div style="flex-grow:1; flex-shrink:1;"></div>
+  </div>
+</div>
  *
  */
 
@@ -399,5 +405,56 @@ namespace Facebook.CSSLayout
             Assert.AreEqual(20, root_child2.LayoutHeight);
         }
 
+        [Test]
+        public void Test_flex_grow_shrink_at_most()
+        {
+            CSSNode root = new CSSNode();
+            root.StyleWidth = 100;
+            root.StyleHeight = 100;
+
+            CSSNode root_child0 = new CSSNode();
+            root.Insert(0, root_child0);
+
+            CSSNode root_child0_child0 = new CSSNode();
+            root_child0_child0.FlexGrow = 1;
+            root_child0_child0.FlexShrink = 1;
+            root_child0.Insert(0, root_child0_child0);
+            root.StyleDirection = CSSDirection.LeftToRight;
+            root.CalculateLayout();
+
+            Assert.AreEqual(0, root.LayoutX);
+            Assert.AreEqual(0, root.LayoutY);
+            Assert.AreEqual(100, root.LayoutWidth);
+            Assert.AreEqual(100, root.LayoutHeight);
+
+            Assert.AreEqual(0, root_child0.LayoutX);
+            Assert.AreEqual(0, root_child0.LayoutY);
+            Assert.AreEqual(100, root_child0.LayoutWidth);
+            Assert.AreEqual(0, root_child0.LayoutHeight);
+
+            Assert.AreEqual(0, root_child0_child0.LayoutX);
+            Assert.AreEqual(0, root_child0_child0.LayoutY);
+            Assert.AreEqual(100, root_child0_child0.LayoutWidth);
+            Assert.AreEqual(0, root_child0_child0.LayoutHeight);
+
+            root.StyleDirection = CSSDirection.RightToLeft;
+            root.CalculateLayout();
+
+            Assert.AreEqual(0, root.LayoutX);
+            Assert.AreEqual(0, root.LayoutY);
+            Assert.AreEqual(100, root.LayoutWidth);
+            Assert.AreEqual(100, root.LayoutHeight);
+
+            Assert.AreEqual(0, root_child0.LayoutX);
+            Assert.AreEqual(0, root_child0.LayoutY);
+            Assert.AreEqual(100, root_child0.LayoutWidth);
+            Assert.AreEqual(0, root_child0.LayoutHeight);
+
+            Assert.AreEqual(0, root_child0_child0.LayoutX);
+            Assert.AreEqual(0, root_child0_child0.LayoutY);
+            Assert.AreEqual(100, root_child0_child0.LayoutWidth);
+            Assert.AreEqual(0, root_child0_child0.LayoutHeight);
+        }
+
     }
 }