[Backport] Fix undefined GYP_MSVS_VERSION failure for non-googlers

Original commit message:
------------------------
This CL adds the missing GYP_MSVS_VERSION environment variable. Without
the patch the following error happen when a non-googler runs
gyp_chromium in Windows.

  File "E:\chromium\src\tools\gyp\pylib\gyp\MSVSVersion.py", line 430, in SelectVisualStudioVersion
    raise ValueError('GYP_MSVS_OVERRIDE_PATH requires GYP_MSVS_VERSION to be '
ValueError: GYP_MSVS_OVERRIDE_PATH requires GYP_MSVS_VERSION to be set to a particular version (e.g. 2010e).

The discussion is found here:
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/XWZ3hZW8Lco

BUG=460462

Review URL: https://codereview.chromium.org/1593313002

build/vs_toolchain.py

@@ -71,6 +71,8 @@ def SetEnvironmentAndGetRuntimeDllDirs():
   elif sys.platform == 'win32' and not depot_tools_win_toolchain:
     if not 'GYP_MSVS_OVERRIDE_PATH' in os.environ:
       os.environ['GYP_MSVS_OVERRIDE_PATH'] = DetectVisualStudioPath()
+    if not 'GYP_MSVS_VERSION' in os.environ:
+      os.environ['GYP_MSVS_VERSION'] = GetVisualStudioVersion()
 
   return vs_runtime_dll_dirs