Enable Python auto-formatting on //components/policy/tools

Causes "git cl format" and "git cl upload" presubmit checks to format
changed lines in a review to adhere to our style guide (PEP-8).

Bug: 846432
Change-Id: I0a91f85df9f69fff7a87868c40479cad671dc892
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989512
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733459}

components/policy/tools/.style.yapf

 [style]
-based_on_style = chromium
+based_on_style = pep8
+
+# New directories should use a .style.yapf that does not include the following:
+column_limit = 80
+indent_width = 2