cc: Move (partially) the predecode image tracking to ImageManager.

cc/tiles/image_manager.h

Index: cc/tiles/image_manager.h
diff --git a/cc/tiles/image_manager.h b/cc/tiles/image_manager.h
index cc6c64fd14f576c78f1dbe2f78875311fde1ec33..046e0948e8bda2775cace12819247934828f59db 100644
--- a/cc/tiles/image_manager.h
+++ b/cc/tiles/image_manager.h
@@ -28,9 +28,13 @@ class CC_EXPORT ImageManager {
       const ImageDecodeController::TracingInfo& tracing_info);
   void UnrefImages(const std::vector<DrawImage>& images);
   void ReduceMemoryUsage();
+  std::vector<scoped_refptr<TileTask>> SetPredecodeImages(
+      std::vector<DrawImage> predecode_images,
+      const ImageDecodeController::TracingInfo& tracing_info);
 
  private:
-  ImageDecodeController* controller_;
+  ImageDecodeController* controller_ = nullptr;
+  std::vector<DrawImage> predecode_locked_images_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageManager);
 };