Use client size for top-level arrange.

Fixes #23.

Perspex.Controls/TopLevel.cs

@@ -299,7 +299,7 @@ namespace Perspex.Controls
                 this.PlatformImpl.ClientSize = finalSize;
             }
 
-            return base.ArrangeOverride(finalSize);
+            return base.ArrangeOverride(this.PlatformImpl.ClientSize);
         }
 
         /// <summary>

Tests/Perspex.Controls.UnitTests/TopLevelTests.cs

@@ -97,6 +97,8 @@ namespace Perspex.Controls.UnitTests
                 Locator.CurrentMutable.RegisterConstant(new LayoutManager(), typeof(ILayoutManager));
 
                 var impl = new Mock<ITopLevelImpl>();
+                impl.SetupProperty(x => x.ClientSize);
+                impl.SetupProperty(x => x.Resized);
 
                 var target = new TestTopLevel(impl.Object)
                 {