Remove optimization in LineBoxList::dirtyLinesFromChangedChild.
Remove optimization in LineBoxList::dirtyLinesFromChangedChild.

There was an optimization to avoid dirtying the next root line box in
certain situations that came from the original patch that added the line
box dirtying system:
https://chromium.googlesource.com/chromium/blink/+/f61897a86d34f6bc6a74b149cb2f921b452f6e9f

Originally it only dirtied the next line box for <br> elements, and then
it slowly get more and more complicated to fix the resulting layout and
security bugs.

Lets try removing this fix and see how bad the perf hit is since it can save
us from the ListHashSet and complicated conditions that have been the result
of correctness and security bugs.

Review URL: https://codereview.chromium.org/1087913006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194087 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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