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>
15 files changed