Gardening: Fix tests failing on Win7(dbg) caused by hitting an assert in FontCache::addSideloadedFontForTesting

NOTRY=true
TBR=arv@chromium.org,tkent@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/549013002

git-svn-id: svn://svn.chromium.org/blink/trunk@181532 bbb929c8-8fbe-4397-9dbb-9b2b20218538