Fix out of flow positioned descendant checks

Fix out of flow positioned descendant checks

With this patch, I've gotten rid of the flaky code for tracking m_hasOutOfFlowPositionedDescendant and replaced it with
  1) A reliable m_hasFixedPositionedDescendant flag, and
  2) a dynamic hasOutOfFlowPositionedDescendant function.

On my z620, the dynamic function took less than 0.3ms in total during a load of gmail. It appears to be of negligible cost, but I've added a trace event to track it, just in case.

NB - this patch depends on https://codereview.appspot.com/8166047/

R=hartmanng@chromium.org,shawnsingh@chromium.org
BUG=196610,196613

[vollick, 2013-03-30 18:39:39]

I am splitting this composite patch into two pieces
(https://codereview.appspot.com/7759045/). This is the second of the two pieces.
The first piece is here (https://codereview.appspot.com/8166047/).

[hartmanng, 2013-04-02 02:02:45]

This patch looks pretty great, IMO. The removal of the optimizations and the
split from from the RAII patch makes this look a _ton_ better than last time I
reviewed it.

https://codereview.appspot.com/8200043/diff/1/Source/WebCore/rendering/Render...
File Source/WebCore/rendering/RenderLayer.cpp (right):

https://codereview.appspot.com/8200043/diff/1/Source/WebCore/rendering/Render...
Source/WebCore/rendering/RenderLayer.cpp:6080: if
(!m_hasFixedPositionedDescendantDirty && m_hasFixedPositionedDescendant)
will this behave correctly if this is dirty? Maybe this should be an assert at
the top of the function?

[vollick, 2013-04-02 13:23:01]

Thanks for the review.

https://codereview.appspot.com/8200043/diff/1/Source/WebCore/rendering/Render...
File Source/WebCore/rendering/RenderLayer.cpp (right):

https://codereview.appspot.com/8200043/diff/1/Source/WebCore/rendering/Render...
Source/WebCore/rendering/RenderLayer.cpp:6080: if
(!m_hasFixedPositionedDescendantDirty && m_hasFixedPositionedDescendant)
On 2013/04/02 02:02:45, hartmanng wrote:
> will this behave correctly if this is dirty? Maybe this should be an assert at
> the top of the function?
Yeah, it will behave correctly, we just won't be able to early out. The fixed
pos element will show up when we search the lists.