This introduces the NodeProperties::ChangeOp helper which guards node
operator changes so that additional checking can be done without any
additional dependencies being pulled into the Node class. For now only
the input count is checked, but additional checking might follow.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1366753003

Cr-Commit-Position: refs/heads/master@{#30916}