Revert of win: Remove unneeded references to visual_studio_version now that it's always 2015.

build/config/win/visual_studio_version.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Path to Visual Studio. If empty, the default is used which is to use the
   # automatic toolchain in depot_tools. If set, you must also set the
   # visual_studio_version and wdk_path.
   visual_studio_path = ""
 
   # Version of Visual Studio pointed to by the visual_studio_path.
-  # Currently always "2015".
+  # Use "2013" for Visual Studio 2013, or "2013e" for the Express version.

[Nico, 2016/11/29 16:23:45]

this is where i would've put the comment

[Michael Achenbach, 2016/11/30 08:18:59]

Ah - yes - that would have been better indeed. Lets just revert this CL soon
again when V8 is on 2015.

   visual_studio_version = ""
 
   # Directory of the Windows driver kit. If visual_studio_path is empty, this
   # will be auto-filled.
   wdk_path = ""
 
   # Full path to the Windows SDK, not including a backslash at the end.
   # This value is the default location, override if you have a different
   # installation location.
   windows_sdk_path = "C:\Program Files (x86)\Windows Kits\10"
 }
 
 if (visual_studio_path == "") {
   toolchain_data =
       exec_script("../../vs_toolchain.py", [ "get_toolchain_dir" ], "scope")
   visual_studio_path = toolchain_data.vs_path
   windows_sdk_path = toolchain_data.sdk_path
   visual_studio_version = toolchain_data.vs_version
   wdk_path = toolchain_data.wdk_dir
   visual_studio_runtime_dirs = toolchain_data.runtime_dirs
 } else {
   assert(visual_studio_version != "",
          "You must set the visual_studio_version if you set the path")
   assert(wdk_path != "",
          "You must set the wdk_path if you set the visual studio path")
   visual_studio_runtime_dirs = []
 }