Wait for store flush in CookieMonster::Delete*Task

Second attempt to commit this, after fixing a flaky test:
http://code.google.com/p/chromium/issues/detail?id=302914

BUG=252217

Review URL: https://codereview.chromium.org/18032002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226720 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed