WebKitNavigationPolicyDecision represents a policy decision for events associated with navigations. If the value of
mouse_button is not 0, then the navigation was triggered
by a mouse event.
If the navigation associated with this policy decision was originally
triggered by a mouse event, this property contains a bitmask of various ModifierType values describing the
modifiers used for that click.
If the navigation associated with this policy decision was originally
triggered by a mouse event, this property contains non-zero button number of the button triggering that event.