Introducing .NET version of css-layout using P/Invoke

Summary:
This version of the css-layout project includes support for .NET projects.  Up in the air is how many configurations of .NET projects we allow for, such as Portable Class Libraries, Universal Windows Platform, .NET Core, etc.  Still needs integration with Buck.
Closes https://github.com/facebook/css-layout/pull/220

Reviewed By: lucasr

Differential Revision: D3909367

Pulled By: emilsjolander

fbshipit-source-id: aaaa9c4ff2d3452649f256c3268cf873cf33a0b9

csharp/Facebook.CSSLayout/MeasureFunction.cs

@@ -0,0 +1,13 @@
+/**
+ * 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.
+ */
+
+namespace Facebook.CSSLayout
+{
+    public delegate void MeasureFunction(CSSNode node, float width, CSSMeasureMode widthMode, float height, CSSMeasureMode heightMode, MeasureOutput measureOutput);
+}