[New Multicolumn] Improve balancing for border/padding and empty block content.

LayoutTests/fast/multicol/balance-trailing-border2-expected.txt

Index: LayoutTests/fast/multicol/balance-trailing-border2-expected.txt
diff --git a/LayoutTests/fast/multicol/balance-trailing-border2-expected.txt b/LayoutTests/fast/multicol/balance-trailing-border2-expected.txt
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+Tests that the bottom border at a column boundary in a block that lives in multiple columns is recorded properly for balancing
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+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
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+PASS mc.offsetHeight is 69
+PASS successfullyParsed is true
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+TEST COMPLETE
+There should be no space between the short black line and the long gray one.
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