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This change is about being more strict about applying style changes in tree
order: first adjust the relationship to the ancestry, THEN adjust the children.

This order is important if a multicol container has a child out-of-flow
multicol container with a spanner, and this child is changed to become in-flow
at the same time as it ceases to be a multicol container, and instead becomes a
spanner. If we change it from multicol to spanner first (instead of making it
part of the outer multicol container first), the outer multicol container is
going to believe that it contains the inner spanner, and we'd end up with a
spanner inside another spanner, which isn't allowed.

#a - multicol
  #b - abspos multicol, changing it to static spanner
    #c - spanner (but it should become a regular block once #b becomes a spanner)

The effect of this fix is that we swap the ordering of notifying the flow thread
about descendant style changes (flowThreadDescendantStyleWillChange(),
flowThreadDescendantStyleDidChange()), compared to when handling style changes
locally on the object (styleWillChange(), styleDidChange()) takes place. More
specifically, we need to get to flowThreadDescendantStyleDidChange() first
(which registers or unregisters descendants in the flow thread - i.e. updates
the LayoutMultiColumnSet / LayoutMultiColumnSpanner placeholder structure), and
THEN to evacuateAndDestroy() (via LayoutBlockFlow::styleDidChange() and 
createOrDestroyMultiColumnFlowThreadIfNeeded()), instead of the other way around.
This way we register #b (now a spanner) in #a first. That will prevent #a from
seeing anything inside #b (spanners are rather opaque).

Since we're now notifying the flow thread from LayoutBox instead of
LayoutObject, we can change the style change notification methods to take
LayoutBox instead of any kind of LayoutObject. The flow thread only cares about
LayoutBox or better here anyway. This allows for some cleanup in the
notification methods, since we no longer need to worry about computed style
weirdness on text layout objects.

BUG=516532
R=eae@chromium.org,jchaffraix@chromium.org,leviw@chromium.org

Review URL: https://codereview.chromium.org/1320843005

git-svn-id: svn://svn.chromium.org/blink/trunk@201993 bbb929c8-8fbe-4397-9dbb-9b2b20218538