Update the VS toolchain hash computation caching.

win_toolchain/get_toolchain_if_necessary.py

Index: win_toolchain/get_toolchain_if_necessary.py
diff --git a/win_toolchain/get_toolchain_if_necessary.py b/win_toolchain/get_toolchain_if_necessary.py
index 37a3207d8836d5840a6630a3afe9f71d0bacd5aa..d7d20b820354009696bee0cdb6c17d49fbeba65b 100755
--- a/win_toolchain/get_toolchain_if_necessary.py
+++ b/win_toolchain/get_toolchain_if_necessary.py
@@ -165,6 +165,10 @@ def CalculateHash(root, expected_hash):
     digest.update(path_without_hash.lower())
     with open(path, 'rb') as f:
       digest.update(f.read())
+
+  # Save the timestamp file if the calculated hash is the expected one.
+  if digest.hexdigest() == expected_hash:
+    SaveTimestampsAndHash(root, digest.hexdigest())
   return digest.hexdigest()