Initialize member variables of ui::ScrollEvent

Initialize member variables of ui::ScrollEvent

With current implementation of GetScrollOffsets on Windows, none values
are changed and remain garbage. To make sure none of this will lead to
crash, just initialize values with zeros.

BUG=

Committed: https://crrev.com/70763eb93a32555910a3b4269aeec51252ab9ec6
Cr-Commit-Position: refs/heads/master@{#360389}

[jsokol, 2015-11-18 15:32:12]

jsokol@opera.com changed reviewers:
+ ananta@chromium.org, sadrul@chromium.org

[jsokol, 2015-11-18 15:35:29]

Hi,

Here's a minor patch we've prepared in Opera when we had discovered several
reports from users on crashes in scroll event handling.
I wasn't able to find similar issue in Chromium and decided not to report a new
one for such trivial change.

Regards,
Jakub Sokół

[sadrul, 2015-11-18 18:17:09]

lgtm

[jsokol, 2015-11-18 18:18:43]

The CQ bit was checked by jsokol@opera.com

[commit-bot: I haz the power, 2015-11-18 18:21:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1461643002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1461643002/1

[commit-bot: I haz the power, 2015-11-18 19:42:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-18 19:44:18]

Patchset 1 (id:??) landed as
https://crrev.com/70763eb93a32555910a3b4269aeec51252ab9ec6
Cr-Commit-Position: refs/heads/master@{#360389}