Update dependencies from https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int build 20200408.6

- Microsoft.DotNet.Wpf.DncEng: 5.0.0-preview.4.20208.4 -> 5.0.0-preview.4.20208.6

eng/Version.Details.xml

@@ -71,9 +71,9 @@
       <Uri>https://github.com/dotnet/runtime</Uri>
       <Sha>59ca590949ade88c712e4ca8c6835acd0e9cbf46</Sha>
     </Dependency>
-    <Dependency Name="Microsoft.DotNet.Wpf.DncEng" Version="5.0.0-preview.4.20208.4">
+    <Dependency Name="Microsoft.DotNet.Wpf.DncEng" Version="5.0.0-preview.4.20208.6">
       <Uri>https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int</Uri>
-      <Sha>3de9ffd307bc8de030788df057e44b9dae78ed38</Sha>
+      <Sha>31cea8e501c4649083b88fd2ec48182106433749</Sha>
     </Dependency>
     <Dependency Name="System.IO.Packaging" Version="5.0.0-preview.4.20205.13" CoherentParentDependency="Microsoft.Private.Winforms">
       <Uri>https://github.com/dotnet/runtime</Uri>

eng/Versions.props

@@ -96,6 +96,6 @@
     <SystemReflectionMetadataLoadContextPackage>System.Reflection.MetadataLoadContext</SystemReflectionMetadataLoadContextPackage>
   </PropertyGroup>
   <PropertyGroup>
-    <MicrosoftDotNetWpfDncEngVersion>5.0.0-preview.4.20208.4</MicrosoftDotNetWpfDncEngVersion>
+    <MicrosoftDotNetWpfDncEngVersion>5.0.0-preview.4.20208.6</MicrosoftDotNetWpfDncEngVersion>
   </PropertyGroup>
 </Project>