Disable flaky AutofillDialogControllerTest.SimulateSuccessfulSignIn on XP.

AutofillDialogControllerTest.SimulateSuccessfulSignIn is flaky on Windows XP:
http://build.chromium.org/p/chromium.win/builders/XP%20Tests%20%283%29/builds/25976
It fails on that run, but then passes on builds 25977, 25978, and 25979.
It might be related to my r244436, I'll disable on XP for now and investigate later.

BUG=333641
TEST=NONE
TBR=dbeam@chromium.org,estade@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/134473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244447 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244447