Bug 1637708 - Convert toolkit.content-background-hang-monitor.disabled to a static pref r=njn

Converts `toolkit.content-background-hang-monitor.disabled` to a static pref. Lets us remove the IsDisabled() call in the BHM.

Differential Revision: https://phabricator.services.mozilla.com/D76376

modules/libpref/init/StaticPrefList.yaml

@@ -8729,6 +8729,12 @@
 # Prefs starting with "toolkit."
 #---------------------------------------------------------------------------
 
+# Returns true if BHR is disabled.
+- name: toolkit.content-background-hang-monitor.disabled
+  type: bool
+  value: false
+  mirror: always
+
 - name: toolkit.scrollbox.horizontalScrollDistance
   type: RelaxedAtomicInt32
   value: 5

modules/libpref/init/all.js

@@ -824,8 +824,6 @@ pref("toolkit.telemetry.unified", true);
 // Extra logging for AsyncShutdown barriers and phases
 pref("toolkit.asyncshutdown.log", false);
 
-pref("toolkit.content-background-hang-monitor.disabled", false);
-
 // Enable JS dump() function.
 // IMPORTANT: These prefs must be here even though they're also defined in
 // StaticPrefList.yaml. They are required because MOZILLA_OFFICIAL is false in

toolkit/components/backgroundhangmonitor/BackgroundHangMonitor.cpp

@@ -16,6 +16,7 @@
 #include "mozilla/LinkedList.h"
 #include "mozilla/Monitor.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_toolkit.h"
 #include "mozilla/Services.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/Telemetry.h"
@@ -607,18 +608,6 @@ bool BackgroundHangMonitor::ShouldDisableOnBeta(const nsCString& clientID) {
   return strtol(suffix, NULL, 16) % BHR_BETA_MOD;
 }
 
-bool BackgroundHangMonitor::IsDisabled() {
-  static bool sPrefCached = false;
-  static bool sPrefCacheValue = false;
-  if (!sPrefCached) {
-    sPrefCached = true;
-    Preferences::AddBoolVarCache(
-        &sPrefCacheValue, "toolkit.content-background-hang-monitor.disabled");
-  }
-
-  return sPrefCacheValue;
-}
-
 bool BackgroundHangMonitor::DisableOnBeta() {
   nsAutoCString clientID;
   nsresult rv =
@@ -643,7 +632,8 @@ void BackgroundHangMonitor::Startup() {
 #ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
   MOZ_ASSERT(!BackgroundHangManager::sInstance, "Already initialized");
 
-  if (XRE_IsContentProcess() && IsDisabled()) {
+  if (XRE_IsContentProcess() &&
+      StaticPrefs::toolkit_content_background_hang_monitor_disabled()) {
     BackgroundHangManager::sDisabled = true;
     return;
   }

toolkit/components/backgroundhangmonitor/BackgroundHangMonitor.h

@@ -135,11 +135,6 @@ class BackgroundHangMonitor {
    */
   static void Shutdown();
 
-  /**
-   * Returns true if BHR is disabled.
-   */
-  static bool IsDisabled();
-
   /**
    * Start monitoring hangs for the current thread.
    *