Use changed_button_flags() when converting MouseEvent to blink::WebMouseEvent.

Use changed_button_flags() when converting MouseEvent to blink::WebMouseEvent.

Event::flags() contains all the buttons that are being held down during event,
while MouseEvent::changed_button_flags() contains the buttons that are
actually pressed/released during the press/release event (and this is what we
really are interested in).

This is a follow up to https://codereview.chromium.org/302593002/ which caused problems
during integration, and it seems to me it got just closed, so I am opening a new one.

Committed: https://crrev.com/a1d1985ccaae302c911676f41c0b062ee7c32371
Cr-Commit-Position: refs/heads/master@{#296691}

[joleksy, 2014-09-22 07:41:58]

joleksy@opera.com changed reviewers:
+ ben@chromium.org

[joleksy, 2014-09-22 07:42:12]

@reviewer: does it look ok to you?

[Ben Goodger (Google), 2014-09-24 17:25:03]

lgtm

[joleksy, 2014-09-25 06:25:50]

The CQ bit was checked by joleksy@opera.com

[commit-bot: I haz the power, 2014-09-25 06:26:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/585393002/1

[commit-bot: I haz the power, 2014-09-25 11:23:06]

Committed patchset #1 (id:1) as 8d0e706bfdeb1f0a382322b536da9bf9ed769146

[commit-bot: I haz the power, 2014-09-25 11:23:47]

Patchset 1 (id:??) landed as
https://crrev.com/a1d1985ccaae302c911676f41c0b062ee7c32371
Cr-Commit-Position: refs/heads/master@{#296691}