Fix behavior of nested click event on label element with checkbox.
static variable value in HTMLLabelElement::defaultEventHandler interferes with handling nested event on another label element.
This cl changes static variable to a class member to remove interference.

BUG=310590
TEST=LayoutTests/fast/events/nested-click-event-on-label.html
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/603393003

git-svn-id: svn://svn.chromium.org/blink/trunk@183243 bbb929c8-8fbe-4397-9dbb-9b2b20218538