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If a node becomes invalid, the cursor snaps to the nearest valid ancestor.
Fixing this uncovered a lot of places where we were assuming that a cursor's
node must be valid, but it's possible that even with this new check that
a cursor could still be invalid, so I added checks in a lot of places.

BUG=613694
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2007183002
Cr-Commit-Position: refs/heads/master@{#397860}