Added System.GC.GetTotalAllocatedBytes to the ref assembly (dotnet/corefx#37635)

Added System.GC.GetTotalAllocatedBytes to the ref assembly (dotnet/corefx#37635)

* Update dependencies from https://github.com/dotnet/coreclr build 20190514.72

- Microsoft.NET.Sdk.IL - 3.0.0-preview6-27714-72
- Microsoft.NETCore.ILAsm - 3.0.0-preview6-27714-72
- Microsoft.NETCore.Runtime.CoreCLR - 3.0.0-preview6-27714-72

* Added GetTotalAllocatedBytes to ref

* Added a test for GC.GetTotalAllocatedBytes

* SImplified the test.

Commit migrated from https://github.com/dotnet/corefx/commit/d0d6a734fc877b76f766548d5e79a63aba5160cf