Marking indexdb tests cursor-continue-validity.html and indexes.html as SLOW on Win7 (dbg)

TBR=zerny@chromium.org
BUG=402379
NOTRY=true

Review URL: https://codereview.chromium.org/462553002

git-svn-id: svn://svn.chromium.org/blink/trunk@179923 bbb929c8-8fbe-4397-9dbb-9b2b20218538