DevTools: do not detach sourcemap in case of style sheet editing
Also the sourcemapping does not make sense for the edited style sheet,
it still seems to be helpful for the web developers as it
gives a brief idea of source file name / approximate line
number.
This was regressed at crrev.com/1806233002
BUG=
257778
R=dgozman
Committed:
https://crrev.com/2dfd4ba707f617a44641c4e732ec986a331b4618
Cr-Commit-Position: refs/heads/master@{#404316}