Test flakiness fix for form submission cancelable

Since test return will be aync, introduced function to test query string return value.

This test fixes isuse introduced by fix for bug196640

R=keishi1, tkent
BUG=353066
TEST=Automated

Review URL: https://codereview.chromium.org/200613004

git-svn-id: svn://svn.chromium.org/blink/trunk@169436 bbb929c8-8fbe-4397-9dbb-9b2b20218538