Introduce toHTMLVideoElement
https://bugs.webkit.org/show_bug.cgi?id=118582

Reviewed by Ryosuke Niwa.

To avoid direct use of static_cast, this patch introduces toHTMLVideoElement.

Source/WebCore:

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::getPluginProxyParams):
* html/HTMLVideoElement.h:
(WebCore::toHTMLVideoElement):
* html/MediaDocument.cpp:
(WebCore::MediaDocumentParser::createDocumentStructure):
(WebCore::descendentVideoElement):
(WebCore::ancestorVideoElement):
* html/shadow/MediaControlElements.cpp:
(WebCore::MediaControlTextTrackContainerElement::updateDisplay):
* rendering/HitTestResult.cpp:
(WebCore::HitTestResult::enterFullscreenForVideo):
* rendering/RenderVideo.cpp:
(WebCore::RenderVideo::videoElement):

Source/WebKit/qt:

* WebCoreSupport/DumpRenderTreeSupportQt.cpp:
(DumpRenderTreeSupportQt::mediaContentUrlByElementId):
* WebCoreSupport/FullScreenVideoQt.cpp:
(WebCore::FullScreenVideoQt::enterFullScreenForNode):
(WebCore::FullScreenVideoQt::exitFullScreenForNode):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152614 268f45cc-cd09-0410-ab3c-d52691b4dbfc