Drop virtual specifier on addRef() and release(). There are no derived
classes which re-implement these functions.
Inlined implementation of constructor and destructor. Destructor is
still virtual, but empty implementation is now visible for derived
classes. Destructor is now protected to make sure that object is
destructed through release().

BUG=angleproject:1263

Change-Id: I744430c1d99b917f3d7b63d47305d937be323ee7
Reviewed-on: https://chromium-review.googlesource.com/320861
Tryjob-Request: Tibor Ouden, den <tibordenouden@gmail.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>