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

Revert of Use changed_button_flags() when converting MouseEvent to blink::WebMouseEvent. (patchset #2 id:20001 of https://codereview.chromium.org/302593002/)

Reason for revert:
Aura builds fail DCHECKs:

ASSERTION FAILED: event.type() == PlatformEvent::MouseMoved || event.button() != NoButton
third_party/WebKit/Source/core/events/MouseEvent.cpp(54)

blink::MouseEvent::create()
blink::Node::dispatchMouseEvent()
blink::EventHandler::dispatchMouseEvent()
blink::EventHandler::handleMousePressEvent()
blink::PageWidgetEventHandler::handleMouseDown()
blink::WebViewImpl::handleMouseDown()
blink::WebViewImpl::handleMouseDown()
blink::PageWidgetDelegate::handleInputEvent()
blink::WebViewImpl::handleInputEvent()
content::RenderWidget::OnHandleInputEvent()

Original issue's description:
> Use changed_button_flags() when converting MouseEvent to blink::WebMouseEvent.
>
> Event::flags() contains all the buttons that are being held down during the event, while MouseEvent::changed_button_flags() contains the buttons that are actually pressed/released during the event (and this is what we really are interested in).
>
> This fixes the issue with LMB being pressed while RMB is being held down. Without the fix, we always get blink::WebMouseEvent::ButtonRight in blink::WebMouseEvent::button.
>
> Committed: https://crrev.com/87e6b0f6d72b12e4b0308b035d1c06f2beb90feb
> Cr-Commit-Position: refs/heads/master@{#293487}

TBR=joleksy@opera.com
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/2591b1df63b910457706206b13d998f1e3069e89
Cr-Commit-Position: refs/heads/master@{#294364}

[Lei Zhang, 2014-09-11 09:33:46]

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

[commit-bot: I haz the power, 2014-09-11 09:34:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/563713002/1

[commit-bot: I haz the power, 2014-09-11 09:35:16]

Committed patchset #1 (id:1) as 38c9e1a43d77914c3309a6c7b37d79e6eeaa3496

[commit-bot: I haz the power, 2014-09-11 09:39:51]

Patchset 1 (id:??) landed as
https://crrev.com/2591b1df63b910457706206b13d998f1e3069e89
Cr-Commit-Position: refs/heads/master@{#294364}