cc: Simplify task and its derived classes.

cc/tiles/gpu_image_decode_controller.h

Index: cc/tiles/gpu_image_decode_controller.h
diff --git a/cc/tiles/gpu_image_decode_controller.h b/cc/tiles/gpu_image_decode_controller.h
index 390ef8ed972cbdc45ccc59cc5efe79c194edbc9f..80587371a6fcfd56019973626bb3f53082bf6b28 100644
--- a/cc/tiles/gpu_image_decode_controller.h
+++ b/cc/tiles/gpu_image_decode_controller.h
@@ -22,7 +22,7 @@ class CC_EXPORT GpuImageDecodeController : public ImageDecodeController {
   // ImageDecodeController overrides.
   bool GetTaskForImageAndRef(const DrawImage& image,
                              uint64_t prepare_tiles_id,
-                             scoped_refptr<ImageDecodeTask>* task) override;
+                             scoped_refptr<Task>* task) override;
   void UnrefImage(const DrawImage& image) override;
   DecodedDrawImage GetDecodedImageForDraw(const DrawImage& draw_image) override;
   void DrawWithImageFinished(const DrawImage& image,
@@ -37,8 +37,7 @@ class CC_EXPORT GpuImageDecodeController : public ImageDecodeController {
   base::Lock lock_;
 
   std::unordered_set<uint32_t> prerolled_images_;
-  std::unordered_map<uint32_t, scoped_refptr<ImageDecodeTask>>
-      pending_image_tasks_;
+  std::unordered_map<uint32_t, scoped_refptr<Task>> pending_image_tasks_;
 };
 
 }  // namespace cc