Update dependencies from https://github.com/dotnet/winforms build 20200528.2 (#3063)

Microsoft.Private.Winforms
 From Version 5.0.0-preview.6.20277.3 -> To Version 5.0.0-preview.6.20278.2
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.6.20277.3">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20278.2">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>aa013a2131343cc1a9dcacbdd955dded4b747046</Sha>
+      <Sha>f4b25306006bb1e416c33bdb052ce697e38b7b8b</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20277.3</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20278.2</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>