add testRunner.resetTestHelperControllers to avoid detecting leak from Source/testing/ inspectors when running leak tests.

This patch also fixes leak-check.js to use <iframe> instead of <frame>.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/22281004

git-svn-id: svn://svn.chromium.org/blink/trunk@155924 bbb929c8-8fbe-4397-9dbb-9b2b20218538