Fix #13983. Switch to //# for sourcemaps.

Normally we'd avoid doing this on a patch release, but Chrome Canary is
spewing deprecation warnings already and that's generating bug tickets.
Change the changelog header style
Revert "Fix #13983. Switch to //# for sourcemaps."

Per the discussion here:
https://code.google.com/p/chromium/issues/detail?id=256636

Thanks to the Chrome team for the quick response!

This reverts commit d53ddc90c1f119fb9148a553443ef3fbc3f3cc99.