Remove RefBase base class from FrameRenderTracker

FrameRenderTracker is never s standalone object, so it's unnecessary.
This avoids allocating a few extra heap objects. More importantly
it no longer interferes with my attempts to understand RefBase
objects that are deallocated without acquiring a strong reference.

Change-Id: I008b04b085fbab1efa4e8948efdcca14b26c247f