Add feature to use percentage as value unit

Summary:
Adds the feature to use percentage as a value unit.

You can use the function ```YGPx(float)``` and ```YGPercent(float)``` for convenience.

I did some benchmarks:

```
Without Percentage Feature - Release x86:

Stack with flex: median: 0.000000 ms, stddev: 0.146683 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.136525 ms
Nested flex: median: 0.000000 ms, stddev: 0.490101 ms
Huge nested layout: median: 23.000000 ms, stddev: 0.928291 ms

Stack with flex: median: 0.000000 ms, stddev: 0.170587 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.143384 ms
Nested flex: median: 0.000000 ms, stddev: 0.477791 ms
Huge nested layout: median: 22.000000 ms, stddev: 2.129779 ms

With Percentage Feature - Release x86:

Stack with flex: median: 0.000000 ms, stddev: 0.132951 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.136525 ms
Nested flex: median: 0.000000 ms, stddev: 0.489570 ms
Huge nested layout: median: 21.000000 ms, stddev: 1.390476 ms
Closes https://github.com/facebook/yoga/pull/258

Reviewed By: dshahidehpour

Differential Revision: D4361945

Pulled By: emilsjolander

fbshipit-source-id: a8f5bc63ad352eb9410d792729e56664468cd76a

java/com/facebook/yoga/YogaValue.java

@@ -0,0 +1,45 @@
+/**
+ * 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.
+ */
+
+package com.facebook.yoga;
+
+import com.facebook.proguard.annotations.DoNotStrip;
+
+@DoNotStrip
+public class YogaValue {
+  static final YogaValue UNDEFINED = new YogaValue(YogaConstants.UNDEFINED, YogaUnit.UNDEFINED);
+  static final YogaValue ZERO = new YogaValue(0, YogaUnit.PIXEL);
+
+  public final float value;
+  public final YogaUnit unit;
+
+  YogaValue(float value, YogaUnit unit) {
+    this.value = value;
+    this.unit = unit;
+  }
+
+  @DoNotStrip
+  YogaValue(float value, int unit) {
+    this(value, YogaUnit.fromInt(unit));
+  }
+
+  @Override
+  public boolean equals(Object other) {
+    if (other instanceof YogaValue) {
+      final YogaValue otherValue = (YogaValue) other;
+      return value == otherValue.value && unit == otherValue.unit;
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return Float.floatToIntBits(value) + unit.intValue();
+  }
+}