Update yapf style from "chromium" to "yapf"

yapf v0.30.0 renamed "chromium" style to "yapf".

Bug: chromium:1306598
Change-Id: I963a7aa5374f743fb10c6cbd398b558b89ae4e96
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/15100
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

.style.yapf

diff --git a/.style.yapf b/.style.yapf
index de0c6a7..fdd0723 100644
--- a/.style.yapf
+++ b/.style.yapf
@@ -1,2 +1,2 @@
 [style]
-based_on_style = chromium
+based_on_style = yapf