Drop the default layout_test batch size for *SAN builds to 40.

BUG=427569
TBR=dpranke@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/726943002

git-svn-id: svn://svn.chromium.org/blink/trunk@185377 bbb929c8-8fbe-4397-9dbb-9b2b20218538