Bug 1540581 - P6. Tidy some C++ declarations in gfx/. r=gerald,jrmuizel

* Remove redundant virtual keywords
* Mark all destructors of inheriting classes as virtual for clarity
* Mark all classes without virtual destructor as final (exposed errors)
* Make destructor virtual where it needed to be (some were missing)
* Replace empty ({}) code declaration in header with = default
* Remove virtual unused methods

I probably missed some, it quickly became a rabbit hole.

Differential Revision: https://phabricator.services.mozilla.com/D26060

gfx/ipc/GPUProcessManager.h

@@ -248,7 +248,7 @@ class GPUProcessManager final : public GPUProcessHost::Listener {
     explicit Observer(GPUProcessManager* aManager);
 
    protected:
-    ~Observer() {}
+    virtual ~Observer() = default;
 
     GPUProcessManager* mManager;
   };