Update yapf style from "chromium" to "yapf"

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

Bug: chromium:1306598
Change-Id: I2506226d92674639cbb9e27adfc281eb27096112
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/104030
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Thiago Perrotta <tperrotta@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
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