Added msvs_target_platform_version and msvs_target_platform_minversion for winrt compilation
With the introduction of Windows 10 Universal applications, winrt app projects require target platform version information. This change allows this value to customized in the GYP.
Patch from coopp@microsoft.com.
R=scottmg@chromium.org
Review URL: https://codereview.chromium.org/1267703004 .