Update dependencies from https://github.com/dotnet/winforms build 20200526.3 (#3051)

Microsoft.Private.Winforms
 From Version 5.0.0-preview.6.20276.1 -> To Version 5.0.0-preview.6.20276.3
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.6.20276.1">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20276.3">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>832abd6652b8d115c3b8f770ecfaf880ccd6940f</Sha>
+      <Sha>d998d38dd919a969bebebf07900c8972b5ab2d88</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20276.1</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20276.3</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>