The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/256093e6ead7db6f7269d8af50a82906891f0caf

commit 256093e6ead7db6f7269d8af50a82906891f0caf
Author: mstensho <mstensho@opera.com>
Date: Fri Oct 07 19:21:03 2016

Don't break before a first in-flow block container.

There's no break opportunity there, but break-inside:avoid used to trick us
into inserting a break there anyway in some cases.

As part of this work, we need allowsPaginationStrut() to check better if a
strut is allowed, or it might just end up getting eaten and forgotten about by
a first in-flow block further up in the tree. This matters for monolithic
content [1], such as lines and image blocks. We should never break inside
those, so allow breaking before them, even if they are the first piece of
content inside some block (just like we did before this change).
break-before-first-line-in-first-child.html and image-block-as-first-child.html
test that we don't regress in this regard.

Also removed a FIXME about checking for sufficient height. This would be
incorrect to fix. If there's no break point here, we have to propagate the
strut, if we're allowed to.

Had to update some tests, and even rename one, because they relied on the old
buggy behavior.

[1] https://drafts.csswg.org/css-break-3/#possible-breaks

BUG= 653690 

Review-Url: https://codereview.chromium.org/2400083003
Cr-Commit-Position: refs/heads/master@{#423926}

[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fast/multicol/break-before-first-line-in-first-child-expected.txt
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fast/multicol/break-before-first-line-in-first-child.html
[modify] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fast/multicol/forced-break-in-nested-columns.html
[modify] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fast/multicol/nested-short-first-row-unsplittable-block.html
[delete] https://crrev.com/96e5488ff8ee752b774b20c3eea3ffd0227a70d6/third_party/WebKit/LayoutTests/fast/multicol/soft-break-before-first-child-expected.txt
[delete] https://crrev.com/96e5488ff8ee752b774b20c3eea3ffd0227a70d6/third_party/WebKit/LayoutTests/fast/multicol/soft-break-before-first-child.html
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/avoid-break-inside-first-child-expected.txt
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/avoid-break-inside-first-child-nested-expected.txt
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/avoid-break-inside-first-child-nested.html
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/avoid-break-inside-first-child.html
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/image-block-as-first-child-expected.txt
[add] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/LayoutTests/fragmentation/image-block-as-first-child.html
[modify] https://crrev.com/256093e6ead7db6f7269d8af50a82906891f0caf/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp