Ignore continuations when collapsing an anonymous block.

When we decide to promote all children of an anonymous block and then
nuke it, ignore any continuation set on the new parent of the children,
to make sure that the children are in fact inserted as children of said
new parent, and not somewhere else in the tree.

This is a hack that's needed as long as we keep the old multicol
implementation. The root cause is probably buggy block continuation
handling, but fixing that would be risky.

BUG=421720

Review URL: https://codereview.chromium.org/656143003

git-svn-id: svn://svn.chromium.org/blink/trunk@183823 bbb929c8-8fbe-4397-9dbb-9b2b20218538