Add compositor support for half-float RGBA buffers and textures

cc/output/direct_renderer.h

Index: cc/output/direct_renderer.h
diff --git a/cc/output/direct_renderer.h b/cc/output/direct_renderer.h
index d08d58d59e71297435ad048252eaea9b8f9e6142..1380a1c01611faadb7f73a55b6ca532f1588ef38 100644
--- a/cc/output/direct_renderer.h
+++ b/cc/output/direct_renderer.h
@@ -144,6 +144,7 @@ class CC_EXPORT DirectRenderer {
 
   // Private interface implemented by subclasses for use by DirectRenderer.
   virtual bool CanPartialSwap() = 0;
+  virtual ResourceFormat BackbufferFormat() const = 0;
   virtual void BindFramebufferToOutputSurface(DrawingFrame* frame) = 0;
   virtual bool BindFramebufferToTexture(DrawingFrame* frame,
                                         const ScopedResource* resource) = 0;