Mark xmlviewer/extensions-api.html as flakily crashing in debug builds

BUG=264501
TBR=haraken

Review URL: https://codereview.chromium.org/20518006

git-svn-id: svn://svn.chromium.org/blink/trunk@154951 bbb929c8-8fbe-4397-9dbb-9b2b20218538