bindings: Retires manual dispatching in createV8{HTML,SVG}Wrapper, etc.

Retires manual dispatching of 'wrap' in createV8{HTML,SVG}Wrapper in
V8{HTML,SVG}ElementWrapperFactory.cpp and its dependents.

This is part of the V8 binding refactoring (Issue 235436) and also
this CL fixes Issue 408160 by removing wrong downcasts by toHTMLUnknownElement().

There are much more code to be removed as part of the refactoring, but let me remove the code in this CL first.  Will continue to remove more unnecessary code.

BUG=408160,235436

Review URL: https://codereview.chromium.org/531183003

git-svn-id: svn://svn.chromium.org/blink/trunk@181440 bbb929c8-8fbe-4397-9dbb-9b2b20218538