This makes it a lot easier to write code that just wants a null PassOwnPtr, especially in
strict PassOwnPtr mode.

Fixes <http://webkit.org/b/59964> Implicit conversion from std::nullptr_t to PassOwnPtr
doesn't work, but should

Reviewed by Adam Barth.

Source/JavaScriptCore:

* wtf/PassOwnPtr.h:
(WTF::PassOwnPtr::PassOwnPtr): Added a non-explicit constructor that takes a nullptr_t.

* wtf/MessageQueue.h:
(WTF::::waitForMessageFilteredWithTimeout):
(WTF::::tryGetMessage):
Use the new implicit conversion.

Source/WebCore:

Take advantage of implicit conversion from nullptr_t to PassOwnPtr

* bindings/js/ScheduledAction.cpp:
* css/CSSStyleSelector.cpp:
* css/MediaList.cpp:
* css/MediaQueryMatcher.cpp:
* css/SVGCSSStyleSelector.cpp:
* dom/MessagePort.cpp:
* html/InputType.cpp:
* html/canvas/WebGLRenderingContext.cpp:
* inspector/InspectorStyleSheet.cpp:
* page/ContextMenuController.cpp:
* page/Page.cpp:
* platform/PlatformGestureRecognizer.cpp:
* platfor...