[master] Update dependencies from dotnet/corefx (#25612)

[master] Update dependencies from dotnet/corefx (#25612)

* Update dependencies from https://github.com/dotnet/corefx build 20190708.6

- Microsoft.NETCore.Platforms - 3.0.0-preview8.19358.6
- Microsoft.Private.CoreFx.NETCoreApp - 4.6.0-preview8.19358.6

* Update dependencies from https://github.com/dotnet/corefx build 20190710.1

- Microsoft.NETCore.Platforms - 3.0.0-preview8.19360.1
- Microsoft.Private.CoreFx.NETCoreApp - 4.6.0-preview8.19360.1

* Update dependencies from https://github.com/dotnet/corefx build 20190710.10

- Microsoft.NETCore.Platforms - 3.0.0-preview8.19360.10
- Microsoft.Private.CoreFx.NETCoreApp - 4.6.0-preview8.19360.10