Fix #2162 - Fixes a crash when javascript's window.open creates a new tab.

[Brandon-T]

There is no need to nil the persistent store when a webView of a different kind is created. We only need to nil it in removeNonPersistentStore when all tabs or all private tabs are killed.

Steps To Reproduce

1. On a real device, enter Private Browsing Mode
2. Open https://www.w3schools.com/jsref/met_win_open.asp
3. Open settings, and enable block all cookies
4. Confirm blocking all cookies.
5. Disable block all cookies from settings.
6. Tap on the try it yourself button on the w3 website
7. It crashes when a new tab is opened from the old tab (popup).

Summary of Changes

This pull request fixes issue #2162
Related to: #1237

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[iccub]

Perfect!

I will make separate ticket for it, to easier track all our crash problems

[iccub]

To whoever merges this: please include the new ticket number(2162) in commit message.

[iccub]

testAllShieldsOff still failing.. merging anyway