This is to make debugging serialization easier by having a way
to inspect the object graph. Serializer::PrintStack() can be
invoked from the debugger to print the current traversal path.

R=jgruber@chromium.org

Change-Id: Ie67408ade7989dc559904821b3f009fdfe3e459f
Reviewed-on: https://chromium-review.googlesource.com/571219Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46662}