Update dependencies from https://github.com/dotnet/winforms build 20190730.6 (#1483)

- Microsoft.Private.Winforms - 4.8.0-preview9.19380.6

eng/Version.Details.xml

 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
-    <Dependency Name="Microsoft.Private.Winforms" Version="4.8.0-preview9.19380.5">
+    <Dependency Name="Microsoft.Private.Winforms" Version="4.8.0-preview9.19380.6">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>1fb126fad97d5293b71c1c8d64f643aa4a00cfff</Sha>
+      <Sha>1a147e1081cc5f172ff6bb60e66e0d997cd6e8ae</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -8,7 +8,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>4.8.0-preview9.19380.5</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>4.8.0-preview9.19380.6</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>