Make raw-pointer iteration in Node safer.
Make raw-pointer iteration in Node safer.

Iterating over nodes with raw pointers is dangerous if the tree is
concurrently modified; which DOM operations can run script can be hard
to predict. This tries to reduce this risk in a couple of ways:

- Disable script execution, or assert that it is already disabled.

- Use const more; move const_cast out of loops.

BUG=225420

Review URL: https://codereview.chromium.org/1381583005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351486}

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