Ignore ubuntu nvprmsaa4/mixed_xfermodes GM result.

BUG=skia:2443

Review URL: https://codereview.chromium.org/653153003
diff --git a/expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86-Release/expected-results.json b/expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86-Release/expected-results.json
index 8c188c7..d3587ad 100644
--- a/expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86-Release/expected-results.json
+++ b/expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86-Release/expected-results.json
@@ -14274,9 +14274,13 @@
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
-          5471873127527415301
+          7591469946375994741
         ]
       ], 
+      "bugs": [
+        2443
+      ], 
+      "ignore-failure": true, 
       "reviewed-by-human": true
     }, 
     "modecolorfilters_565.png": {