Change DontCheck under-invalidation checking mode to CacheSkipper

Source/platform/graphics/paint/DrawingDisplayItem.h

Index: Source/platform/graphics/paint/DrawingDisplayItem.h
diff --git a/Source/platform/graphics/paint/DrawingDisplayItem.h b/Source/platform/graphics/paint/DrawingDisplayItem.h
index b73b21e9b2962d9d3879c44f72205d7ba325e410..3e2b721d5b64062928c3557cee82b8d5903152bb 100644
--- a/Source/platform/graphics/paint/DrawingDisplayItem.h
+++ b/Source/platform/graphics/paint/DrawingDisplayItem.h
@@ -19,7 +19,6 @@ public:
     enum UnderInvalidationCheckingMode {
         CheckPicture, // Check if the new picture and the old picture are the same
         CheckBitmap, // Check if the new picture and the old picture produce the same bitmap
-        DontCheck // Skip for known bugs. Each usage should have a FIXME and a link to the bug.
     };
 #endif