Fix default value for DefaultFileSystemWriteGuardSetting

Users can change it, so it isn't always 3

Change-Id: I71b9cb834ba79414b7590a07601b58d2edb49f39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547305Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828856}

components/policy/resources/policy_templates.json

@@ -6227,7 +6227,7 @@
         'per_profile': True,
       },
       'example_value': 2,
-      'default': 3,
+      'default': None,
       'id': 774,
       'caption': '''Control use of the File System API for writing''',
       'tags': ['website-sharing'],