The "doh-rollout.self-enabled" pref is not removed after changing the "network.trr.mode" pref value via “about:preferences#networking” page

[SoftVision-CarmenFat]

[Affected Platforms]:

• All Windows

[Affected Versions]:

• Firefox Release 70.0.1, Build ID 20191030021342

[Prerequisites]:

• Have an active Normandy testing recipe for DoH.
• You are enrolled in the Treatment branch.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Navigate to "about:preferences#networking" page.
3. Click the "Settings" button and uncheck the "Enable DNS over HTTPS" checkbox.
4. Click the "OK" button.
5. Restart the browser.
6. Observe if the "doh-rollout.self-enabled" preference is removed.

[Expected result]:

• The "doh-rollout.self-enabled" pref is removed.

[Actual result]:

• The "doh-rollout.self-enabled" pref is displayed.

[Notes]:

• The issue is not reproducible when the user chooses to opt-out from the experiment through the DoH doorhanger.
• I have attached a screen recording of the issue:

[maxxcrawford]

This pref is cleared on the SECOND restart after user disabling DoH manually. This add-on only fires its main function (and makes changes) retroactively after Start Up/Network Change/doh-rollout.enabled pref update.

To make this more transparent, I've written out the chain of events that happen what a user manually disables DoH (via about:preferences), including expected prefs that should update/remain the same. Note that there are additional prefs being set (all under the doh-rollout. namespace) but the prefs outlined below are the ones that should be checked in this test case.

Step 1: First Run Start with a clean profile. Run add-on however you prefer. (Via flipping doh-rollout.enabled pref, or Normandy study.)

Expected Prefs:

• doh-rollout.self-enabled: true
• doh-rollout.disable-heuristics: Should not exist
• network.trr.mode: 2

Step 2: Update DoH via about:preferences _When a user unchecks DoH in about:preferences#networking, this triggers NO actions by the add-on immediately. The add-on only runs on browser startup, network change event (note the 60s timeout here) or when the doh-rollout.enabled pref is updated.

Expected Prefs:

• doh-rollout.self-enabled: true
• doh-rollout.disable-heuristics: Should not exist
• network.trr.mode: 0

Step 3: Restart/Network Change Addon detects that user pref has changed for network.trr.mode. Heuristics will be disabled and DoH is NOT enabled.

Expected Prefs:

• doh-rollout.self-enabled: true
• doh-rollout.disable-heuristics: true
• network.trr.mode: 0

Step 4: Restart/Network Change The add-on is now in its final disable loop, detecting/catching the doh-rollout.disable-heuristics pref. This logic will clear the doh-rollout.self-enabled pref and not run heuristics.

Expected Prefs:

• doh-rollout.self-enabled: Should not exist
• doh-rollout.disable-heuristics: true
• network.trr.mode: 0

[maxxcrawford]

However, I think we could safely clear this pref whenever rememberDisableHeuristics() is ran, to make the pref removed a step earlier.