fix up GM suppressions

BUG=skia:2911

Review URL: https://codereview.chromium.org/538723002
diff --git a/expectations/gm/ignored-tests.txt b/expectations/gm/ignored-tests.txt
index 9cbcdb7..52a1a79 100644
--- a/expectations/gm/ignored-tests.txt
+++ b/expectations/gm/ignored-tests.txt
@@ -43,13 +43,14 @@
 fontcache
 
 # mtklein
+downsamplebitmap_checkerboard_high_512_256
 downsamplebitmap_checkerboard_low_512_256
 downsamplebitmap_checkerboard_medium_512_256
 downsamplebitmap_checkerboard_none_512_256
-downsamplebitmap_image_high_mandrill_512
-downsamplebitmap_image_low_mandrill_512
-downsamplebitmap_image_medium_mandrill_512
-downsamplebitmap_image_none_mandrill_512
+downsamplebitmap_image_high_mandrill_512.png
+downsamplebitmap_image_low_mandrill_512.png
+downsamplebitmap_image_medium_mandrill_512.png
+downsamplebitmap_image_none_mandrill_512.png
 downsamplebitmap_text_high_72.00pt
 downsamplebitmap_text_low_72.00pt
 downsamplebitmap_text_medium_72.00pt