Fixed incorrect usage of ASSERT_ENABLED macro.

Caused undesired work to be done in release builds.

BUG=393857

Review URL: https://codereview.chromium.org/395633008

git-svn-id: svn://svn.chromium.org/blink/trunk@178140 bbb929c8-8fbe-4397-9dbb-9b2b20218538