Do not reduce the number of layout_test workers in *SAN builds.

This is a partial revert of r184607. Turns out reducing the number of workers
was not the right solution for our performance issues.

BUG=427569
R=dpranke@chromium.org
NOTRY=true
TEST=test-webkitpy

Review URL: https://codereview.chromium.org/722193002

git-svn-id: svn://svn.chromium.org/blink/trunk@185303 bbb929c8-8fbe-4397-9dbb-9b2b20218538