Change call-sites now that SkCanvas is not ref-counted

cc/playback/discardable_image_map.h

Index: cc/playback/discardable_image_map.h
diff --git a/cc/playback/discardable_image_map.h b/cc/playback/discardable_image_map.h
index adae86248074cf55fec472076a52369a56fe49b0..00506809a5fcba8869307187813fbd9bd8a72a4a 100644
--- a/cc/playback/discardable_image_map.h
+++ b/cc/playback/discardable_image_map.h
@@ -36,7 +36,7 @@ class CC_EXPORT DiscardableImageMap {
 
    private:
     DiscardableImageMap* image_map_;
-    sk_sp<SkCanvas> metadata_canvas_;
+    std::unique_ptr<SkCanvas> metadata_canvas_;
   };
 
   DiscardableImageMap();
@@ -51,7 +51,7 @@ class CC_EXPORT DiscardableImageMap {
   friend class ScopedMetadataGenerator;
   friend class DiscardableImageMapTest;
 
-  sk_sp<SkCanvas> BeginGeneratingMetadata(const gfx::Size& bounds);
+  std::unique_ptr<SkCanvas> BeginGeneratingMetadata(const gfx::Size& bounds);
   void EndGeneratingMetadata();
 
   std::vector<std::pair<DrawImage, gfx::Rect>> all_images_;