Fix touch handler ASSERT when an iframe containing a plugin changes source

When an iframe's src is changed, Document::removeAllEventListeners is used to 
clear all event listeners in the document, as a result the parent document
will clear out any known touch handler for the iframe.  But if the iframe
contained a plugin, then when that plugin is destroyed it'll try to
notify the document that it is no longer handling touch events, and (if it's the
only touch event handler on the document) the parent document will get a duplicate
removal notification.

Also, despite Document::removeAllEventListeners telling it's parent document
it has no more handlers, it now actually clears it's own list too.  

BUG=254203

Review URL: https://chromiumcodereview.appspot.com/22301016

git-svn-id: svn://svn.chromium.org/blink/trunk@155801 bbb929c8-8fbe-4397-9dbb-9b2b20218538