Mark storage/indexeddb/pending-activity.html slow in Win/Mac Debug

Also, consolidate expectations for storage/indexeddb/objectstore-cursor

BUG=243782
TBR=jochen@chromium.org
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/19305003

git-svn-id: svn://svn.chromium.org/blink/trunk@154252 bbb929c8-8fbe-4397-9dbb-9b2b20218538