Move basic virtual methods from Element.cpp to Element.h

This matches the normal style and also lets the compiler make certain callers
inline if the class is FINAL. This is part of the quest to make Element.cpp
shorter and Element.h more self documenting (ex. default virtual boolean methods
have the values be obvious).

Review URL: https://chromiumcodereview.appspot.com/19991002

git-svn-id: svn://svn.chromium.org/blink/trunk@154735 bbb929c8-8fbe-4397-9dbb-9b2b20218538