This CL adds a mechanism for wrapping C++ objects to Gin. The approach in this
CL is similar to Blink's ScriptWrappable class, with a couple of differences:

1) gin::Wrappable has a vtable whereas Blink's ScriptWrappable class does not.
   Having a vtable in this base class lets us simplify a large number of
   concerns. We've talked about adding a vtable to ScriptWrappable but have
   avoided it because Blink creates many thousands of wrapped objects. When we
   refactor Blink to use Gin, we can still support the non-vtable approach, but
   most clients of Gin will want the simpler approach.

2) In Gin, we've bound together the notion of being reference counted with the
   notion of being wrappable from JavaScript. In Blink, those concepts are
   separate because we don't want to introduce a virtual destructor for
   ScriptWrappable. However, because gin::Wrappable already has a vtable,
   adding a virtual destructor is relatively cheap.

Actually wrapping a C++ object still takes too much typing, but we can improve
that in future CLs.

R=jochen@chromium.org
BUG=317398

Review URL: https://codereview.chromium.org/79203004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236555 0039d316-1c4b-4281-b951-d872f2087c98