Fix dev tools hack.

This fixes the problem hacked around in
41fe7e28aa. In the end the problem wasn't
ScrollViewer, it was that the correct size wasn't being passed to
Window.Arrange.

Perspex.Diagnostics/DevTools.cs

@@ -118,10 +118,8 @@ namespace Perspex.Diagnostics
             {
                 Window window = new Window
                 {
-                    // HACK: Set width and height here as a quick fix as there's a problem with 
+                    Width = 1024,
-                    // the dev tools window hanging when it's set to auto-size.
+                    Height = 512,
-                    Width = 800,
-                    Height = 600,
                     Content = new DevTools
                     {
                         Root = (Window)sender,

Perspex.Layout/LayoutManager.cs

@@ -206,9 +206,9 @@ namespace Perspex.Layout
 
                 this.toArrange = new Heap<Item>(HeapType.Minimum);
 
-                if (!this.Root.IsArrangeValid)
+                if (!this.Root.IsArrangeValid && this.Root.IsMeasureValid)
                 {
-                    this.Root.Arrange(new Rect(this.Root.ClientSize));
+                    this.Root.Arrange(new Rect(this.Root.DesiredSize.Value));
                 }
 
                 if (this.toMeasure.Count > 0)