Fix py3 compat issue in get_toolchain_if_necessary.

Bug: 1207012
Change-Id: I6ee0cb6be8e86490ec0ecf5a4f2e236546bb92ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2900993Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>

win_toolchain/get_toolchain_if_necessary.py

@@ -223,7 +223,7 @@ def SaveTimestampsAndHash(root, sha1):
     'sha1': sha1,
   }
   with open(MakeTimestampsFileName(root, sha1), 'wb') as f:
-    json.dump(timestamps_data, f)
+    f.write(json.dumps(timestamps_data).encode('utf-8'))
 
 
 def HaveSrcInternalAccess():