Delete ASSERT_DISABLED

ASSERT_ENABLED doens't have more not double negatives.

Note the #ifndef ASSERT_DISABLED in DatabaseThread.cpp was always compiled out
because ASSERT_DISABLED was always defined to be either 0 or 1.

Review URL: https://codereview.chromium.org/331793004

git-svn-id: svn://svn.chromium.org/blink/trunk@176120 bbb929c8-8fbe-4397-9dbb-9b2b20218538