Oilpan: Ensure that left-most vtable is the first thing to be initialized for polymorphic classes with trace methods.
    
This is done by ensuring one of two properties for any polymorphic class with a trace method.
1. If the trace method is virtual, then the left-most base class must define a virtual trace method too.
2. If the trace method is non-virtual, then the left-most base class must define some virtual method.
    
The Blink GC plugin statically checks these properties.

Review URL: https://codereview.chromium.org/463543002

git-svn-id: svn://svn.chromium.org/blink/trunk@180281 bbb929c8-8fbe-4397-9dbb-9b2b20218538