show grey insecure icon for passive mixed content

[diracdeltas]

fix #2168

Auditors: @darkdh

Test Plan:

1. automated tests related to lockIcon should pass
2. go to mixed.badssl.com; the icon should be a grey unlocked icon
3. clicking on the icon should tell you that the page is partially insecure

• 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).

[darkdh]

++