Don't adjust the amount of content_shells run for layout tests

It's unclear whether this actually improves anything or just slows us
down.

R=dpranke@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22519003

git-svn-id: svn://svn.chromium.org/blink/trunk@155679 bbb929c8-8fbe-4397-9dbb-9b2b20218538