Issue #1360 - Part 1: Simplify layers acceleration prefs.

This gets rid of platform-dependent hard-coded defaults, but keeps
build-time blocking if there is no GL provider (in which case layers
acceleration almost certainly won't work because it needs a GL
compositor and would likely crash without)

New prefs are
- layers.acceleration.enabled to enable HWA
- layers.acceleration.force to force it enabled (requires .enabled to be
  set as well)

This is the platform part of this issue. The rest will be front-end work
(Preference UI integration and pref migration)

gfx/thebes/gfxPlatform.h

@@ -698,7 +698,7 @@ protected:
     virtual already_AddRefed<mozilla::gfx::VsyncSource> CreateHardwareVsyncSource();
 
     // Returns whether or not layers should be accelerated by default on this platform.
-    virtual bool AccelerateLayersByDefault();
+    bool AccelerateLayersByDefault();
 
     // Returns a prioritized list of available compositor backends for acceleration.
     virtual void GetAcceleratedCompositorBackends(nsTArray<mozilla::layers::LayersBackend>& aBackends);