Moving the declaration of operator<< from objects-printer.cc to
heap-object.h makes sure that it is visible.

Change-Id: I316db9c03a464974129b8e9c776423bb80066cdd
Reviewed-on: https://chromium-review.googlesource.com/c/1382737Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58350}