win: update the toolchain if necessary

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 3b3fb1af50e234343df286bf8c68c0dbcc697b76..0ebf1f5f461617c4ed951d336c6e1d33ad2102a9 100644
--- a/DEPS
+++ b/DEPS
@@ -217,6 +217,12 @@ hooks = [
     'action': ['python', 'src/tools/clang/scripts/update.py', '--if-needed'],
   },
   {
+    # Update the Windows toolchain if necessary.
+    'name': 'win_toolchain',
+    'pattern': '.',
+    'action': ['python', 'src/build/vs_toolchain.py', 'update'],
+  },
+  {
     # Update LASTCHANGE. This is also run by export_tarball.py in
     # src/tools/export_tarball - please keep them in sync.
     'name': 'lastchange',