Only allow forced fragmentainer breaks at class A break points.

third_party/WebKit/LayoutTests/fast/multicol/forced-break-before-complex-margin-collapsing-expected.txt

-Given a parent block with a forced break before, and with an empty collapse-through child followed by a child with a top margin, check that the parent block still ends up at the exact top of the next column. The collapse-through child prevents us from correctly estimating the logical top of the parent initially.
+Given a parent block with a forced break before, and with an empty collapse-through child followed by a child with a top margin, check that the parent block is affected by the top margin of its second child. The collapse-through child prevents us from correctly estimating the logical top of the parent initially.
 
-Below there should be a yellow box. Inside it there should be a bottom-left aligned blue box and a top-center aligned green box.
+Below there should be a yellow box. There should be no red. In the first column, the word "first" should be seen once. In the second column, the word "second" should be seen twice. In the third column, the word "third" should be seen twice.
 
 
 
 
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-
-
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+first
+second
+second
+third
+third
 PASS