When looking for adjacent content inside multicols, don't consider inner multicols.

When looking for adjacent content inside multicols, don't consider inner multicols.

When walking the tree backwards for a suitable object, we need to check that we
didn't end up inside an inner multicol container. Walking forwards is more
trivial (no changes there), but added an assertion that we haven't entered an
inner multicol just in case anyway.

Also added some assertions which will fail if
nextInPreOrderAfterChildrenSkippingOutOfFlow() or
previousInPreOrderSkippingOutOfFlow() misbehave.

Need to check with shouldSkipInsertedOrRemovedChild() before calling those two
functions now, since they are not meant to be called on children that are to be
skipped (especially important for column sets and spanner placeholders, which
should always be skipped).

BUG=498431, 498630
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196957

[dsinclair, 2015-06-11 15:38:23]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-06-11 15:38:24]

lgtm

https://codereview.chromium.org/1180613005/diff/1/Source/core/layout/LayoutMu...
File Source/core/layout/LayoutMultiColumnFlowThread.cpp (right):

https://codereview.chromium.org/1180613005/diff/1/Source/core/layout/LayoutMu...
Source/core/layout/LayoutMultiColumnFlowThread.cpp:120: for (ancestor =
object->parent(); ; ancestor = ancestor->parent()) {
Is there no possibility for this to walk up and ancestor == nullptr?

[commit-bot: I haz the power, 2015-06-11 15:38:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1180613005/1

[commit-bot: I haz the power, 2015-06-11 15:41:42]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196957

[mstensho (USE GERRIT), 2015-06-11 16:22:13]

https://codereview.chromium.org/1180613005/diff/1/Source/core/layout/LayoutMu...
File Source/core/layout/LayoutMultiColumnFlowThread.cpp (right):

https://codereview.chromium.org/1180613005/diff/1/Source/core/layout/LayoutMu...
Source/core/layout/LayoutMultiColumnFlowThread.cpp:120: for (ancestor =
object->parent(); ; ancestor = ancestor->parent()) {
On 2015/06/11 15:38:24, dsinclair wrote:
> Is there no possibility for this to walk up and ancestor == nullptr?

Shouldn't be. |object| is something we get to via
previousInPreOrder(flowThread).