Make WebNode::to<Type>() (and toConst) casts contain type asserts.

third_party/WebKit/Source/web/WebElement.cpp

Index: third_party/WebKit/Source/web/WebElement.cpp
diff --git a/third_party/WebKit/Source/web/WebElement.cpp b/third_party/WebKit/Source/web/WebElement.cpp
index 0a9668ee9266eace187c508674462091c4cc31b9..dda7e0905afa6f37ba727d1b8d161b36121972f9 100644
--- a/third_party/WebKit/Source/web/WebElement.cpp
+++ b/third_party/WebKit/Source/web/WebElement.cpp
@@ -180,6 +180,8 @@ WebElement::WebElement(const PassRefPtrWillBeRawPtr<Element>& elem)
 {
 }
 
+DEFINE_WEB_NODE_TYPE_CASTS(WebElement, isElementNode());
+
 WebElement& WebElement::operator=(const PassRefPtrWillBeRawPtr<Element>& elem)
 {
     m_private = elem;