Oilpan: add [WillBeGarbageCollected] for Element.

Annotate Element as being a heap allocated object
with Oilpan enabled, and generate binding layer code
which assumes so for Element and its derived interface
objects.

To accommodate this, bindings code generated for
a named constructor need to consult the "gc_type" of
its interface, and no longer not assume RefCounted.
Along similar lines, custom element processing can no
longer assume that the elements will be RefCounted-based
either.

R=haraken@chromium.org,tkent@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/290333008

git-svn-id: svn://svn.chromium.org/blink/trunk@174537 bbb929c8-8fbe-4397-9dbb-9b2b20218538