Bug 1198196 - rework EVENTLOOP_UI_LAG_EXP_MS to record all lag. r=vladan

Previously we were only logging if we accumulated 50ms of lag. Start logging
all lag so we can use this measure to compare smaller changes in UI
responsiveness.

toolkit/components/telemetry/Histograms.json

@@ -4244,14 +4244,13 @@
     "releaseChannelCollection": "opt-out",
     "description": "Whether the homepage was imported during browser migration. Only available on release builds during firstrun."
   },
-  "EVENTLOOP_UI_LAG_EXP_MS": {
+  "EVENTLOOP_UI_ACTIVITY_EXP_MS": {
     "alert_emails": ["perf-telemetry-alerts@mozilla.com"],
+    "bug_numbers": [1198196],
     "expires_in_version": "never",
     "kind": "exponential",
-    "low": 50,
     "high": "60000",
-    "n_buckets": 20,
-    "extended_statistics_ok": true,
+    "n_buckets": 50,
     "description": "Widget: Time it takes for the message before a UI message (ms)"
   },
   "FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS": {

xpcom/threads/HangMonitor.cpp

@@ -370,16 +370,10 @@ NotifyActivity(ActivityType aActivityType)
   // penalties here.
   gTimestamp = PR_IntervalNow();
 
-  // If we have UI activity we should reset the timer and report it if it is
-  // significant enough.
+  // If we have UI activity we should reset the timer and report it
   if (aActivityType == kUIActivity) {
-    // The minimum amount of lag time that we should report for telemetry data.
-    // Mozilla's UI responsiveness goal is 50ms
-    static const uint32_t kUIResponsivenessThresholdMS = 50;
-    if (cumulativeUILagMS > kUIResponsivenessThresholdMS) {
-      mozilla::Telemetry::Accumulate(mozilla::Telemetry::EVENTLOOP_UI_LAG_EXP_MS,
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::EVENTLOOP_UI_ACTIVITY_EXP_MS,
                                      cumulativeUILagMS);
-    }
     cumulativeUILagMS = 0;
   }