DevTools: highlight network project files with respect to canonical mime type

Currently, we rely on uiSourceCode extension to figure out an appropriate
highlighter. This turns out to be wrong approach for the files originating
from network project - they should be highlighted with respect to the
mimeType in their HTTP headers.

BUG=411863
R=vsevik, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/574043002

git-svn-id: svn://svn.chromium.org/blink/trunk@182417 bbb929c8-8fbe-4397-9dbb-9b2b20218538