Reduce the number of layout_test workers we use by default for *SAN builds.

It looks like we might be overloading the machines by default. Ultimately
we might want some way to adjust the scaling factor from the command line
but this is a temporary hack to play with things for now to help determine
what works best.

R=earthdok@chromium.org
BUG=427569

Review URL: https://codereview.chromium.org/688093003

git-svn-id: svn://svn.chromium.org/blink/trunk@184607 bbb929c8-8fbe-4397-9dbb-9b2b20218538