Column balancing support in the region based multicol implementation.

Column balancing support in the region based multicol implementation.

Start by setting the column height to the flow thread's height divided
by the number of columns. Then stretch the column height until
contents fits without creating overflowing columns, or until the
maximum allowed column height is reached, whichever comes first. This
may require several layout iterations (but normally only a
couple). For each time we stretch, stretch by the least amount
required to make a difference to which box goes where.

Also make sure that the columns get tall enough to honor orphans
and widows settings.

This change takes some tiny steps in preparing for multiple column
set support.

This patch is ported from WebKit.
https://bugs.webkit.org/show_bug.cgi?id=116033
Reviewed in WebKit by David Hyatt.

BUG=252675
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=153028

[mstensho (USE GERRIT), 2013-06-21 13:23:39]

Moving this code from WebKit to Blink went rather smoothly, but there was one
thing:

RenderBox::computeContentAndScrollbarLogicalHeightUsing() has been changed in
Blink to take two arguments, instead of one. The second argument is "intrinsic
height", which in my case is unknown at the time of calling this method. So I
decided to let -1 mean unknown. I'm only interested in non-auto and
non-intrinsic heights here. I use it to set column heights and maximum column
heights. If there are better suggestions, let me know.

Reviewers picked rather randomly; feel free to suggest others. :) This has
already been approved for WebKit by David Hyatt, though.

[eseidel, 2013-06-25 21:02:21]

lgtm

[commit-bot: I haz the power, 2013-06-25 21:02:30]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mstensho@opera.com/16943008/1

[commit-bot: I haz the power, 2013-06-25 21:52:58]

Change committed as 153028