Reland "Always use the Win 10 SDK, and optionally detect/use the latest version"

This is a reland of 6da1d32c16a0598e9b32a4e5b60f99aca871a399
Original change's description:
> Always use the Win 10 SDK, and optionally detect/use the latest version
> 
> For bot toolchain builds, we were already using the Win 10 SDK. Local
> 2015 builds were using a strange hybrid of Win 10 and Win 8 SDKs. Now
> we always use the Win 10 SDK exclusively.
> 
> This adds two new GN arguments: win_sdk, which points at the top level
> Windows 10 SDK directory, and win_sdk_version, which can be used to
> pin to a specific version. (Otherwise we use a script to detect the
> most recent version).
> 
> Bug: skia:
> Change-Id: I5aabb5eb9e7f483e3676b67b50356ddd0421cf7d
> Reviewed-on: https://skia-review.googlesource.com/81304
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> Commit-Queue: Brian Osman <brianosman@google.com>

Bug: skia:
Change-Id: Ide0d6bd3413c4fe7a8bada7d3d32bdba9709d11b
Reviewed-on: https://skia-review.googlesource.com/81501
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
15 files changed