Disable flaky BackgroundKeepaliveActive on Windows

This CL disables AppBackgroundPageNaClTest.BackgroundKeepaliveActive on
Windows because of flakiness.

BUG=346278
TBR=scheib@chromium.org

Review URL: https://codereview.chromium.org/177093003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252911 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252911