Update ignore file

NOTRY=True
TBR=
BUG=skia:

Review URL: https://codereview.chromium.org/844813004
diff --git a/expectations/gm/ignored-tests.txt b/expectations/gm/ignored-tests.txt
index 01322ca..7bc03a3 100644
--- a/expectations/gm/ignored-tests.txt
+++ b/expectations/gm/ignored-tests.txt
@@ -35,6 +35,7 @@
 
 # egdaniel
 colortype_xfermodes
+mixed_xfermodes
 
 # robertphillips - skia:2995
 blurrects