View page source should respect private windows

[Sh1d0w]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fixes #4069

@bbondy

Test Plan:
Open a private tab and load any website
Right click on the page and choose Show Source

[bbondy]

nice catch, thanks.

[da2x]

Please also test reloading the tab with the source … .