Avoid scrollbar construction/destruction thrashing during flex layout.

Avoid scrollbar construction/destruction thrashing during flex layout.

BUG=528940
R=skobes@chromium.org,pdr@chromium.org

***
Re-landing this change after it was reverted; original patch:

https://codereview.chromium.org/1357423007
***

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.

TBR=skobes,haraken,keishi

Committed: https://crrev.com/86dda5bf4e56074eab86f12cbff115fed315f15e
Cr-Commit-Position: refs/heads/master@{#351472}

[szager1, 2015-09-30 00:11:36]

The CQ bit was checked by szager@chromium.org

[commit-bot: I haz the power, 2015-09-30 00:14:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1373413002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1373413002/1

[commit-bot: I haz the power, 2015-09-30 00:23:14]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-30 00:23:15]

Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  win_chromium_x64_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[szager1, 2015-09-30 00:38:23]

The CQ bit was checked by szager@chromium.org

[commit-bot: I haz the power, 2015-09-30 00:40:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1373413002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1373413002/1

[commit-bot: I haz the power, 2015-09-30 02:15:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-30 02:16:30]

Patchset 1 (id:??) landed as
https://crrev.com/86dda5bf4e56074eab86f12cbff115fed315f15e
Cr-Commit-Position: refs/heads/master@{#351472}

[MuVen, 2015-10-01 12:41:59]

On 2015/09/30 02:16:30, commit-bot: I haz the power wrote:
> Patchset 1 (id:??) landed as
> https://crrev.com/86dda5bf4e56074eab86f12cbff115fed315f15e
> Cr-Commit-Position: refs/heads/master@{#351472}

Skobes@,
This CL has broken scrollbars/custom-scrollbar-changing-style.html testcase
https://codereview.chromium.org/1308173005/ . Shall i raise a bug for the
same or reuse 522389.

[MuVen, 2015-10-01 13:41:48]

sataya.m@samsung.com changed reviewers:
+ sataya.m@samsung.com

[MuVen, 2015-10-01 13:41:48]

https://codereview.chromium.org/1373413002/diff/1/third_party/WebKit/Source/c...
File third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
(right):

https://codereview.chromium.org/1373413002/diff/1/third_party/WebKit/Source/c...
third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp:683:
else if (box().style()->overflowX() == OSCROLL)
small typo error, corrected at 
https://codereview.chromium.org/1381943002/