[Browser tests] Move "I can click the X on the save box" to shared_steps.rb

Doesn't belong in headings_steps.rb because it's used in other
*.feature files like bullets.feature

Change-Id: I23d3772653140e6e4639785c3622fc9a2b446801
Merge "[Browser tests] Move "I can click the X on the save box" to shared_steps.rb"