If you dragged the node out of the bounding box, this commit allows you to see
it again after you zoom. The zoom looks jittery, but I think it's better than
not being able to see it at all.

BUG=
R=danno

Review-Url: https://codereview.chromium.org/2168713005
Cr-Commit-Position: refs/heads/master@{#38024}