Use pep8 instead of chromium yapf python style

Chromium's style guide uses pep8 as its yapf formatting
style. The chromium yapf style appears to be the outdated style. So use
pep8 instead of chromium.

Style guide: https://chromium.googlesource.com/chromium/src/+/master/styleguide/python/python.md

Change-Id: I44e7825373fe0052f13e8cd2e515cbe9ed53fa86
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2172435
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Auto-Submit: Philip Pfaffe <pfaffe@chromium.org>

.style.yapf

@@ -1,2 +1,2 @@
 [style]
-based_on_style = chromium
+based_on_style = pep8