[gn build] Make sync_source_lists_from_cmake handle one-line sources lines

sync_source_lists_from_cmake now also looks for source files in
`sources += [ "foo.cc" ]` lines, which allows us to remove most
`# Make `gn format` not collapse this` comments.

(sync_source_lists_from_cmake doesn't look for `foo_headers += [...]`
still, so the comment is still needed in two places for that.)

No intentional behavior change.