[C++] arrow::io::CompressedOutputStream::raw() impementation is missing

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #2990

_{Note: This issue was originally created as ARROW-3835. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2990
#2990