Ignore failure of mixed_xfermodes_gpu on N4

BUG=skia:1596
NOTRY=true
R=epoger@google.com, bsalomon@google.com

Author: borenet@google.com

Review URL: https://codereview.chromium.org/190843004

git-svn-id: http://skia.googlecode.com/svn/trunk@13707 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed