Bug 1539031 - (part 3) Set the GC modes from prefs with the new mode r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D24851

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

dom/base/nsJSEnvironment.cpp

@@ -2357,12 +2357,18 @@ static void SetMemoryGCModePrefChangedCallback(const char* aPrefName,
       Preferences::GetBool("javascript.options.mem.gc_incremental");
   JSGCMode mode;
   if (enableIncrementalGC) {
+    if (enableZoneGC) {
       mode = JSGC_MODE_ZONE_INCREMENTAL;
-  } else if (enableZoneGC) {
+    } else {
+      mode = JSGC_MODE_INCREMENTAL;
+    }
+  } else {
+    if (enableZoneGC) {
       mode = JSGC_MODE_ZONE;
     } else {
       mode = JSGC_MODE_GLOBAL;
     }
+  }
 
   SetGCParameter(JSGC_MODE, mode);
 }