Fix offset issue of background-attachment:fixed on scroller

Fix offset issue of background-attachment:fixed on scroller

We should not include scroll offset of the scroller itself with the
fixed background in http://crrev.com/cc8fa7dd73e.

BUG=568625
TEST=paint/overflow/fixed-background-on-scroller-scroll.html

Committed: https://crrev.com/70a919c39f5d644c7676f9c76686166826dccc8d
Cr-Commit-Position: refs/heads/master@{#371310}

[Xianzhu, 2016-01-25 18:35:44]

wangxianzhu@chromium.org changed reviewers:
+ pdr@chromium.org, trchen@chromium.org

[pdr., 2016-01-25 19:13:51]

LGTM

[Xianzhu, 2016-01-25 19:39:22]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2016-01-25 19:43:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1630693003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1630693003/1

[commit-bot: I haz the power, 2016-01-25 20:54:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-01-25 20:55:40]

Description was changed from

==========
Fix offset issue of background-attachment:fixed on scroller

We should not include scroll offset of the scroller itself with the
fixed background in http://crrev.com/cc8fa7dd73e.

BUG=568625
TEST=paint/overflow/fixed-background-on-scroller-scroll.html
==========

to

==========
Fix offset issue of background-attachment:fixed on scroller

We should not include scroll offset of the scroller itself with the
fixed background in http://crrev.com/cc8fa7dd73e.

BUG=568625
TEST=paint/overflow/fixed-background-on-scroller-scroll.html

Committed: https://crrev.com/70a919c39f5d644c7676f9c76686166826dccc8d
Cr-Commit-Position: refs/heads/master@{#371310}
==========

[commit-bot: I haz the power, 2016-01-25 20:55:41]

Patchset 1 (id:??) landed as
https://crrev.com/70a919c39f5d644c7676f9c76686166826dccc8d
Cr-Commit-Position: refs/heads/master@{#371310}