[printing] Use colon as separator

Drive-by-fixes:
- brief print prototype

Bug: v8:7310
Change-Id: I4df82bd76c05ff48a5860bd14cf7711140c0b062
Reviewed-on: https://chromium-review.googlesource.com/923737
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51329}