Tell git_cl.py to use "pdfium:" bug prefix in CL descriptions.

A PDFium CL should have a prefix for the Bug: entry, to avoid ambiguity
as to which project the bug is for. The prefix should either be pdfium
or chromium in most cases. Default to pdfium.

Change-Id: I6b3b4a6df45990319d6af01fa402572c5cef8269
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/72132
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

codereview.settings

diff --git a/codereview.settings b/codereview.settings
index 77265c9..0cc32b9 100644
--- a/codereview.settings
+++ b/codereview.settings
@@ -1,4 +1,5 @@
 # This file is used by git cl to get repository specific information.
+BUG_PREFIX: pdfium:
 CC_LIST: pdfium-reviews@googlegroups.com
 CODE_REVIEW_SERVER: codereview.chromium.org
 GERRIT_HOST: True