Indicate that UserDataPolicy could be platform_only=False in the future

This policy doesn't have to be platform_only=True

Bug: 1140118
Change-Id: I5c4f3432137c9e65e3c8173bbb24ce953577f7a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485753Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818556}

components/policy/resources/policy_templates.json

@@ -3539,6 +3539,8 @@
       'features': {
         'dynamic_refresh': False,
         'per_profile': False,
+        # TODO(crbug.com/1149118): Query cloud policies before using
+        # UserDataDir so this policy won't need to be platform_only anymore.
         'platform_only': True,
       },
       'example_value': '${users}/${user_name}/Chrome',