cc: Simplify task and its derived classes.

cc/tiles/software_image_decode_controller.h

Index: cc/tiles/software_image_decode_controller.h
diff --git a/cc/tiles/software_image_decode_controller.h b/cc/tiles/software_image_decode_controller.h
index 4e959c7f2f8deae238e8e1e1bd9b4435c12e29ff..b5ed096887950653e271aa11fd9a5280a13f8d31 100644
--- a/cc/tiles/software_image_decode_controller.h
+++ b/cc/tiles/software_image_decode_controller.h
@@ -105,7 +105,7 @@ class CC_EXPORT SoftwareImageDecodeController : 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& image) override;
   void DrawWithImageFinished(const DrawImage& image,
@@ -199,7 +199,7 @@ class CC_EXPORT SoftwareImageDecodeController : public ImageDecodeController {
   // more of them.
   bool CanHandleImage(const ImageKey& key);
 
-  std::unordered_map<ImageKey, scoped_refptr<ImageDecodeTask>, ImageKeyHash>
+  std::unordered_map<ImageKey, scoped_refptr<Task>, ImageKeyHash>
       pending_image_tasks_;
 
   // The members below this comment can only be accessed if the lock is held to