Blink: Enable Python auto-formatting

This enables clang-format-style autoformatting via YAPF.

Bug: 846432
Change-Id: I212c731c53a7cd2b8da839f30688c5c23d3b7c8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989232
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729944}

third_party/blink/.style.yapf

+[style]
+based_on_style = pep8
+column_limit = 132