tools/gn: Implement Windows SDK version command line switch

Windows SDK command line switch enables ability to browse source code
smoothly in Visual Studio using various installed Windows SDK versions.

BUG=634788

Change-Id: I64af058a0ccfd10a189cda61938a9ec7b5f9c60d
Reviewed-on: https://chromium-review.googlesource.com/535639
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480307}

• AUTHORS[diff]
• tools/gn/command_gen.cc[diff]
• tools/gn/visual_studio_writer.cc[diff]
• tools/gn/visual_studio_writer.h[diff]
• tools/gn/visual_studio_writer_unittest.cc[diff]