A lot of deverlopers use deprecated sourceURL syntax. We should add console warning message before removing this.
Original CL: https://codereview.chromium.org/1495633002/

BUG=chromium:558998
LOG=Y
R=yangguo@chromium.org,hablich@chromium.org,adamk@chromium.org

Review URL: https://codereview.chromium.org/1654223003

Cr-Commit-Position: refs/heads/master@{#33709}