[MD settings] fix filter on return from sub-page

This CL resets the filter_ member in the site data handler (c++) so that
the filter will be reapplied when revisiting the page.

Bug: 820816
Change-Id: Id710d8f6b9ec35f8ee69088dbbd8d8d9b3691c38
Reviewed-on: https://chromium-review.googlesource.com/964986Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543555}

chrome/browser/ui/webui/settings/settings_cookies_view_handler.cc

@@ -333,6 +333,7 @@ void CookiesViewHandler::HandleReloadCookies(const base::ListValue* args) {
 
   AllowJavascript();
   cookies_tree_model_.reset();
+  filter_.clear();
   sorted_sites_.clear();
   EnsureCookiesTreeModelCreated();
 }