[New Multicolumn] Use the term "content run" in favor of "forced break".

[New Multicolumn] Use the term "content run" in favor of "forced break".

You get one content run even without any forced breaks.

Documentation improvements. In particular for flow thread layout and
column balancing.

Also don't assume that a column set starts at flow thread block offset 0.
This is only true for the first set. So far we only have one set, but this
will soon change, with the introduction of support for column-span:all.
Check with the flow thread portion rectangle to get the correct start offset.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173879

[Julien - ping for review, 2014-05-12 10:30:06]

lgtm

[commit-bot: I haz the power, 2014-05-12 10:30:19]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mstensho@opera.com/267373005/1

[commit-bot: I haz the power, 2014-05-12 11:44:47]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/7217)

[commit-bot: I haz the power, 2014-05-12 12:57:48]

Change committed as 173879