Use SECURITY_ASSERT_ENABLED.

Double negative like "#if !ASSERT_DISABLED" is hard to understand, and
additional "|| defined(ADDRESS_SANITIZER)" makes much harder.

BUG=none
TEST=none; no behavior changes
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/146403002

git-svn-id: svn://svn.chromium.org/blink/trunk@165748 bbb929c8-8fbe-4397-9dbb-9b2b20218538