Update dependencies from https://github.com/dotnet/winforms build 20200520.1 (#3028)

Microsoft.Private.Winforms
 From Version 5.0.0-preview.6.20269.8 -> To Version 5.0.0-preview.6.20270.1
Co-authored-by: Ndotnet-maestro[bot] <dotnet-maestro[bot]@users.noreply.github.com>

eng/Version.Details.xml

 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
-    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20269.8">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20270.1">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>50720fb1dee457dcdc80cebe392d98d6dd00a00e</Sha>
+      <Sha>b107541e07055d32d7f0248c2d886efb2301e98e</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20269.8</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20270.1</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>