The ornamental design for washing machine drum lifter, as shown and described.
1. Washing machine drum lifter
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
1.8 : Reference
The broken lines indicate portions of the article that form no part of the claimed design; the dot-dashed lines in 1.8 represent boundary lines of drum so that the washing machine drum lifter inside drum can be seen and form no part of the claimed design.