Forcing vs_toolchain update only on a win checkout.

Bug: webrtc:8875
Change-Id: I2a6c73dc6a80641343a42141bd2c259db594910e
Reviewed-on: https://webrtc-review.googlesource.com/52343
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21998}
diff --git a/DEPS b/DEPS
index d23873d..8f9a647 100644
--- a/DEPS
+++ b/DEPS
@@ -248,7 +248,8 @@
     # Update the Windows toolchain if necessary. Must run before 'clang' below.
     'name': 'win_toolchain',
     'pattern': '.',
-    'action': ['python', 'src/build/vs_toolchain.py', 'update'],
+    'condition': 'checkout_win',
+    'action': ['python', 'src/build/vs_toolchain.py', 'update', '--force'],
   },
   {
     # Update the Mac toolchain if necessary.