Start removing the double-negative !ASSERT_DISABLED

This CL is a simple s/if !ASSERT_DISABLED/if ASSERT_ENABLED/.
Subsequent CLs will clean up the remaining uses and then delete the macro.

NOTRY=true
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/327323002

git-svn-id: svn://svn.chromium.org/blink/trunk@175987 bbb929c8-8fbe-4397-9dbb-9b2b20218538