Fix C# delegate calling conventions

Summary:
When using CSS-Layout in a C# UWP project in x86, by default the MSVC compiler defaults the delegate calling convention to cdecl, while .NET assumes that all delegates are declared using stdcall.  This causes a problem when invoking such as this error:

```
Run-Time Check Failure #0 - The value of ESP was not properly saved across
a function call.  This is usually a result of calling a function declared with one
calling convention with a function pointer declared with a different calling
convention.
```

This PR changes the calling convention in the C# code to reflect cdecl by using the `UnmanagedFunctionPointer` attribute and setting the calling convention to `CallingConvention.Cdecl`.

```csharp
 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
  public delegate CSSSize CSSMeasureFunc(
        IntPtr context,
        float width,
        CSSMeasureMode widthMode,
        float height,
        CSSMeasureMode heightMode);
```

I have updated all calls as well to other functions.  I
Closes https://github.com/facebook/css-layout/pull/231

Reviewed By: emilsjolander

Differential Revision: D4063437

Pulled By: splhack

fbshipit-source-id: b1069a1b9f675d2623a64a1c5f3189292a18a646

csharp/Facebook.CSSLayout/CSSAssert.cs

@@ -8,11 +8,13 @@
  */
 
 using System;
+using System.Runtime.InteropServices;
 
 namespace Facebook.CSSLayout
 {
     internal static class CSSAssert
     {
+        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
         public delegate void FailFunc(string message);
 
         private static bool _assertInitialized;