https://bugs.webkit.org/show_bug.cgi?id=98007

Reviewed by Hajime Morita.

Description of bug:
If the mouse left button is pressed on a spin button in
input[type=number] and a 'change' event handler opens a modal dialog
such as alert(), a repeating timer doesn't stop and mouse event
capturing isn't released even though the mouse pointer isn't on the spin
button.
A user will see repeating alert dialogs for a document like <input
type=number value=1 onchange="if (this.value==1) {alert(...);
this.value=1;}"> by clicking the up button.

How to solve:
We should notify modal dialog or popup open to a spin button.
This patch introduce PopupOpeningObserver. Chrome notifies it when
any dialogs / popups is opening. SpinButtonElement implements
PopupOpeningObserver and registers/unregisters itself to/from
Chrome.

No new tests. This is a behavior change, but it's very hard to make an
automated test for timer-related behavior.

* page/PopupOp...