Update the VS toolchain hash computation caching.

R=hans@chromium.org, scottmg@chromium.org
BUG=495204

Review-Url: https://codereview.chromium.org/2052533002

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()