cc: Drop locked decodes from checker image tracker on cache switch.

cc/tiles/checker_image_tracker.h

Index: cc/tiles/checker_image_tracker.h
diff --git a/cc/tiles/checker_image_tracker.h b/cc/tiles/checker_image_tracker.h
index 335050c6b9df5ec861be30f2f6b58f852bbebf07..64560fb774804b39d9262c336cef479842bd1ef3 100644
--- a/cc/tiles/checker_image_tracker.h
+++ b/cc/tiles/checker_image_tracker.h
@@ -66,6 +66,10 @@ class CC_EXPORT CheckerImageTracker {
   // raster.
   void DisallowCheckeringForImage(const PaintImage& image);
 
+  bool has_locked_decodes_for_testing() const {
+    return !image_id_to_decode_.empty();
+  }
+
  private:
   enum class DecodePolicy {
     // The image can be decoded asynchronously from raster. When set, the image