Add medium image quality to software predecode.

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 fe55d88b670bdff9bb42be2ca73ae4115610681f..7fbeccb8cb2d694fbe95579b81000ca60bf7d732 100644
--- a/cc/tiles/software_image_decode_controller.h
+++ b/cc/tiles/software_image_decode_controller.h
@@ -211,6 +211,14 @@ class CC_EXPORT SoftwareImageDecodeController
   DecodedDrawImage GetDecodedImageForDrawInternal(const ImageKey& key,
                                                   const DrawImage& draw_image);
 
+  // GetMediumQualityImageDecode is called by DecodeImageInternal to get the
+  // decoded and scaled image for medium quality.
+  // Like DecodeImageInternal, it should be called with no lock acquired
+  // and it also returns nullptr if the decoding or scaling failed.
+  std::unique_ptr<DecodedImage> GetMediumQualityImageDecode(
+      const ImageKey& key,
+      const SkImage& image);
+
   // GetOriginalImageDecode is called by DecodeImageInternal when the quality
   // does not scale the image. Like DecodeImageInternal, it should be called
   // with no lock acquired and it returns nullptr if the decoding failed.
@@ -229,13 +237,6 @@ class CC_EXPORT SoftwareImageDecodeController
   void RefAtRasterImage(const ImageKey& key);
   void UnrefAtRasterImage(const ImageKey& key);
 
-  // These functions indicate whether the images can be handled and cached by
-  // ImageDecodeController or whether they will fall through to Skia (with
-  // exception of possibly prerolling them). Over time these should return
-  // "false" in less cases, as the ImageDecodeController should start handling
-  // more of them.
-  bool CanHandleImage(const ImageKey& key);
-
   // Helper function which dumps all images in a specific ImageMRUCache.
   void DumpImageMemoryForCache(const ImageMRUCache& cache,
                                const char* cache_name,
@@ -259,11 +260,6 @@ class CC_EXPORT SoftwareImageDecodeController
 
   MemoryBudget locked_images_budget_;
 
-  // Note that this is used for cases where the only thing we do is preroll the
-  // image the first time we see it. This mimics the previous behavior and
-  // should over time change as the compositor starts to handle more cases.
-  std::unordered_set<uint32_t> prerolled_images_;
-
   ResourceFormat format_;
 
   // Used to uniquely identify DecodedImages for memory traces.