Properly detect active password manager in contentSettings

[bsclifton]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Ran git rebase -i to squash commits if needed.

Previous code would fail for anyone who had a session before PW manager was simplified down to a single choice, because they did not have ACTIVE_PASSWORD_MANAGER set.

Auditors: @dandart

Fixes #3516

[bbondy]

pls write out tests and tag with manual steps specified for the issue. Pls make sure to try the tests :) And also ping Alex explicitly to try the steps. I want to make sure we're putting an end to this thread of bugs.

[bbondy]

++ merged

[bsclifton]

Before verifying the fix, please make sure you have an account created here:
https://trac.torproject.org/projects/tor/login

To verify the fix

Case where PW manager was set but wasn't respected after upgrade

• Ensure all Brave instances are closed
• Download session-store-1_bsc0.11.1.zip (0.11.1 session; 1Password enabled)
• Unzip / move this session file to the appropriate location (Mac is ~/Libraries/Application\ Support/brave/), making sure to rename it as session-store-1
• Install a version of Brave which has the fix
• Launch Brave and navigate to Preferences > Security
• Notice that 1Password is selected
• Navigate to https://trac.torproject.org/projects/tor/login
• Login using credentials you have
• You will NOT be prompted to remember this user/pass, as expected (1Password may prompt you).

[bsclifton]

cc: @alexwykoff to try the steps sometime tomorrow