Stringification of Json wrongly used quotes for "true", "false" and
"null".

Drive-by: Manually flush std::cout when sending messages. This might
fix the server on windows.

R=tebbi@chromium.org

Bug: v8:8880
Change-Id: Ie499595a1b429514c5d8b1d3ece24f4690ece02e
Reviewed-on: https://chromium-review.googlesource.com/c/1498132Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59992}