Do not destroy Helper Plugin page during stopActiveDOMObjects().

Destroying the page, specifically calling frameDetached() can cause JavaScript
to execute, which is not allowed within stopActiveDOMObjects(), which gets called while destroying the main page in WebViewImpl::close().
The result is a RELEASE_ASSERT().
To prevent this, defer the closeHelperPlugin() call until later in WebViewImpl::close().
For consistency with other cases, closeHelperPluginSoon() always defers closure to a Timer. WebViewImpl::close() will cancel the Timer and close the Helper Plugins immediately; otherwise the Timer-initiated WebHelperPluginImpl::closeHelperPlugin() call would access deleted objects.

BUG=262980,263634,269193

Review URL: https://chromiumcodereview.appspot.com/23154002

git-svn-id: svn://svn.chromium.org/blink/trunk@156237 bbb929c8-8fbe-4397-9dbb-9b2b20218538