Ensure that removing an iframe from the DOM tree disconnects its Frame

SubframeLoadingDisabler wasn't catching the case when an <iframe> was, in its unload
handler, removed and re-added to the same parent. Fix this by using a count of
SubframeLoadingDisablers that are on the stack for a given root, rather than a simple
boolean (using a HashCountedSet instead of a HashSet).

BUG=260375

Review URL: https://chromiumcodereview.appspot.com/21887005

git-svn-id: svn://svn.chromium.org/blink/trunk@156174 bbb929c8-8fbe-4397-9dbb-9b2b20218538