Oilpan: disconnected scrollbars must be externally observable as such.

Oilpan: disconnected scrollbars must be externally observable as such.

Followup the Scrollbar and ScrollableArea Oilpan changes in r198064, and
have explicit disconnects of Scrollbars from their ScrollableAreas be
observable via the Scrollbar afterwards.

The assumption made in r198064 that no one wanted to observe Scrollbars
in that detached/disconnected state, doesn't hold. SelectorChecker and
AXScrollbar are two examples.

R=haraken
BUG=504655
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198144

[sof, 2015-07-01 13:31:27]

sigbjornf@opera.com changed reviewers:
+ oilpan-reviews@chromium.org

[sof, 2015-07-01 13:31:27]

please take a look.

For, 


https://codereview.chromium.org/1215973002/diff/20001/Source/platform/scroll/...

I wrongly concluded that disconnects wouldn't have to be observable, they do.
Addresses 


http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=fas...

[haraken, 2015-07-01 13:38:29]

LGTM

Not related to this issue, but I'm still wondering why it's ok not to have
ASSERT(!m_scrollableArea) in Scrollbar's destructor. If the disconnected
scrollbars must be observable, I guess Scrollbar::disconnectFromScrollableArea()
must be called for all disconnected scrollbars.

[sof, 2015-07-01 13:48:57]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-07-01 13:49:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1215063006/1

[commit-bot: I haz the power, 2015-07-01 14:21:38]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198144