Also set LIBPATH in Windows toolchain environment

Needed to find *.winmd files for UWP build.

This CL and crrev.com/c/2007881 were used to generate the recent
toolchains with UWP support:
b92fff97f2e0323e99803f37f4b77b843bdbf69d in crrev.com/c/2015984 and
9ff60e43ba91947baca460d0ca3b1b980c3a2c23 in crrev.com/c/2025528.

Bug: chromium:1032635
Change-Id: I1ab0b6e11a30600deb377078815c268d8f9027e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2013579
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

win_toolchain/package_from_installed.py

@@ -277,6 +277,10 @@ def GenerateSetEnvCmd(target_dir):
     ['..', '..'] + vc_tools_parts + ['include'],
     ['..', '..'] + vc_tools_parts + ['atlmfc', 'include'],
   ])
+  libpath_dirs = [
+    ['..', '..'] + vc_tools_parts + ['lib', 'x86', 'store', 'references'],
+    ['..', '..', 'win_sdk', 'UnionMetadata', WIN_VERSION],
+  ]
   # Common to x86, x64, and arm64
   env = collections.OrderedDict([
     # Yuck: These have a trailing \ character. No good way to represent this in
@@ -284,6 +288,7 @@ def GenerateSetEnvCmd(target_dir):
     ('VSINSTALLDIR', [['..', '..\\']]),
     ('VCINSTALLDIR', [['..', '..', 'VC\\']]),
     ('INCLUDE', include_dirs),
+    ('LIBPATH', libpath_dirs),
   ])
   # x86. Always use amd64_x86 cross, not x86 on x86.
   env['VCToolsInstallDir'] = [['..', '..'] + vc_tools_parts[:]]