Bug 1602903 - Collect per-window useRemoteSubframes telemetry, r=mconley

This telemetry records, for each toplevel window, the relationship between the
actual fission-enabled status of the window, and the "fission.autostart" pref.

This takes the form of a categorized telemetry histogram with the following
keys:
 * EnabledByAutostart:  useRemoteSubframes == true   fission.autostart == true
 * EnabledByUser:       useRemoteSubframes == true   fission.autostart == false
 * DisabledByAutostart: useRemoteSubframes == false  fission.autostart == false
 * DisabledByUser:      useRemoteSubframes == false  fission.autostart == true

'DisabledByUser' helps indicate useful information such as how often fission
testers need to open non-fission windows due to breakage, while
'EnabledByAutostart' and 'EnabledByUser' helps give a better image of how many
people are testing fission.

Differential Revision: https://phabricator.services.mozilla.com/D61927

--HG--
extra : moz-landing-system : lando

browser/base/content/browser.js

@@ -2303,6 +2303,8 @@ var gBrowserInit = {
       NewTabPagePreloading.maybeCreatePreloadedBrowser(window);
     });
 
+    scheduleIdleTask(reportRemoteSubframesEnabledTelemetry);
+
     if (AppConstants.NIGHTLY_BUILD) {
       scheduleIdleTask(() => {
         FissionTestingUI.init();
@@ -9401,6 +9403,21 @@ var ConfirmationHint = {
   },
 };
 
+function reportRemoteSubframesEnabledTelemetry() {
+  let autostart = Services.prefs.getBoolPref("fission.autostart");
+
+  let categoryLabel = gFissionBrowser ? "Enabled" : "Disabled";
+  if (autostart == gFissionBrowser) {
+    categoryLabel += "ByAutostart";
+  } else {
+    categoryLabel += "ByUser";
+  }
+
+  Services.telemetry
+    .getHistogramById("WINDOW_REMOTE_SUBFRAMES_ENABLED_STATUS")
+    .add(categoryLabel);
+}
+
 if (AppConstants.NIGHTLY_BUILD) {
   var FissionTestingUI = {
     init() {

toolkit/components/telemetry/Histograms.json

@@ -15069,5 +15069,20 @@
     "n_buckets": 100,
     "bug_numbers": [1597956],
     "description": "milliseconds to complete a TLS session resumption with external cache"
+  },
+  "WINDOW_REMOTE_SUBFRAMES_ENABLED_STATUS": {
+    "record_in_processes": ["main"],
+    "products": ["firefox"],
+    "alert_emails": ["nika@mozilla.com"],
+    "expires_in_version": "82",
+    "kind": "categorical",
+    "labels": [
+      "EnabledByAutostart",
+      "EnabledByUser",
+      "DisabledByAutostart",
+      "DisabledByUser"
+    ],
+    "bug_numbers": [1602903],
+    "description": "Fission enabled status of individual windows in fission.autostart and non-fission.autostart profiles"
   }
 }