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

Review-Url: https://codereview.chromium.org/2116903002
Cr-Commit-Position: refs/heads/master@{#404316}
7 files changed