Update dependencies from https://github.com/dotnet/winforms build 20200615.2

Microsoft.Private.Winforms
 From Version 5.0.0-preview.7.20311.7 -> To Version 5.0.0-preview.7.20315.2

eng/Version.Details.xml

 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
-    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.7.20314.3">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.7.20315.2">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>32eaa68bf5cde8a8619b61d079f48ce96f31cfd5</Sha>
+      <Sha>6a7e0d7278953c2c9119c4f0fd414bef234f247b</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.7.20314.3</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.7.20315.2</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>