Navigate back when possible from malicious error page

[laghee]

Task/Issue URL: https://app.asana.com/0/0/1209289574297313/f

Description

Take users to previous URL on Leave Site button press when history exists instead of always closing tab and opening a new one.

Steps to test this PR

Feature 1

• Open a new tab and accumulate some safe site history
• Go to https://privacy-test-pages.site/security/badware/malware.html in the same tab
• Tap Leave Site and check that tab goes back to last safe URL instead of closing and opening a new tab
• Open a new tab and directly go to https://privacy-test-pages.site/security/badware/malware.html
• Tap Leave Site and check that tab is closed and a fresh one is opened