Revert of Avoid scrollbar construction/destruction thrashing during flex layout.

Revert of Avoid scrollbar construction/destruction thrashing during flex layout. (patchset #4 id:60001 of https://codereview.chromium.org/1357423007/ )

Reason for revert:
Broke oilpan:

../../third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h:71:5: error: [blink-gc] Class 'ScrollbarManager' contains invalid fields.
    class ScrollbarManager {
    ^
../../third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h:110:9: note: [blink-gc] Member field 'm_hBar' in unmanaged class declared here:
        RefPtrWillBeMember<Scrollbar> m_hBar;
        ^
../../third_party/WebKit/Source/platform/heap/Handle.h:919:28: note: expanded from macro 'RefPtrWillBeMember'
#define RefPtrWillBeMember blink::Member
                           ^
../../third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h:111:9: note: [blink-gc] Member field 'm_vBar' in unmanaged class declared here:
        RefPtrWillBeMember<Scrollbar> m_vBar;
        ^
../../third_party/WebKit/Source/platform/heap/Handle.h:919:28: note: expanded from macro 'RefPtrWillBeMember'
#define RefPtrWillBeMember blink::Member
                           ^
1 error generated.

Original issue's description:
> Avoid scrollbar construction/destruction thrashing during flex layout.
>
> BUG=528940
> R=skobes@chromium.org,pdr@chromium.org
>
> Before this change:
>
> https://codereview.chromium.org/1295933003
>
> ... blocks with overflow:auto would delay updating their scrollbars until
> after all flex layout was finished.  After the change, scrollbar info is
> updated immediately during the course of layout.  Flex items may run
> layout multiple times during flex layout; if a flex item has auto scrollbars,
> it may create and destroy its scrollbars multiple times.
>
> Aside from being a performance problem, this can cause WebScrollbarThemePainter
> to point to a stale Scrollbar object.  If a flex item has scrollbars prior to
> layout; then the flex item destroys and creates scrollbars during layout; and
> at the end of layout, it still has a scrollbar; then
> CompositedDeprecatedPaintLayerMapping::updateOverflowControlsLayers will not
> update the WebScrollbarThemePainter with the final Scrollbar object.
>
> We could fix this in updateOverflowControlsLayers, but that wouldn't address
> the performance issue of needlessly creating and destroying scrollbars during
> flex layout.  This patch avoids destroying scrollbars that are no longer
> deemed necessary, until after all flexing is finished.
>
> Committed: https://crrev.com/662a21f7bbebca99b8d968a8104da7fb53af820c
> Cr-Commit-Position: refs/heads/master@{#351130}

TBR=pdr@chromium.org,skobes@chromium.org,szager@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=528940
Committed: https://crrev.com/1a157eb11dda435aefcad65d3e49ecc36112c02b
Cr-Commit-Position: refs/heads/master@{#351223}

[Lei Zhang, 2015-09-29 01:13:11]

Created Revert of Avoid scrollbar construction/destruction thrashing during flex
layout.

[commit-bot: I haz the power, 2015-09-29 01:13:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1370373003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1370373003/1

[commit-bot: I haz the power, 2015-09-29 01:15:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 01:16:31]

Patchset 1 (id:??) landed as
https://crrev.com/1a157eb11dda435aefcad65d3e49ecc36112c02b
Cr-Commit-Position: refs/heads/master@{#351223}

[Lei Zhang, 2015-09-29 03:06:26]

So I reverted because oilpan bots were on the main waterfall and they were red.
Oilpan team have said that's ok and they will fix it. So feel free to
un-reverted.

[szager1, 2015-09-29 04:10:54]

OK, thanks for letting me know.

On Mon, Sep 28, 2015 at 8:06 PM <thestig@chromium.org> wrote:

> So I reverted because oilpan bots were on the main waterfall and they were
> red.
> Oilpan team have said that's ok and they will fix it. So feel free to
> un-reverted.
>
> https://codereview.chromium.org/1370373003/
>

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[Lei Zhang, 2015-09-29 08:45:08]

But since you know this CL breaks oilpan, please try to fix it rather than
reverting the revert.