Bug 1571660 - Tidy Zone class definition r=sfink

Patch to move all data members together at the start of the class and a couple of other minor tidyups.

Differential Revision: https://phabricator.services.mozilla.com/D62592

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

js/src/gc/Zone.cpp

@@ -136,11 +136,17 @@ JS::Zone::Zone(JSRuntime* rt)
       // ProtectedData checks in CheckZone::check may read this field.
       helperThreadUse_(HelperThreadUse::None),
       helperThreadOwnerContext_(nullptr),
-      uniqueIds_(this),
-      suppressAllocationMetadataBuilder(this, false),
       arenas(this),
-      tenuredAllocsSinceMinorGC_(0),
       types(this),
+      data(this, nullptr),
+      tenuredStrings(this, 0),
+      tenuredBigInts(this, 0),
+      allocNurseryStrings(this, true),
+      allocNurseryBigInts(this, true),
+      isSystem(this, false),
+      suppressAllocationMetadataBuilder(this, false),
+      uniqueIds_(this),
+      tenuredAllocsSinceMinorGC_(0),
       gcWeakMapList_(this),
       compartments_(),
       crossZoneStringWrappers_(this),
@@ -156,19 +162,10 @@ JS::Zone::Zone(JSRuntime* rt)
       functionToStringCache_(this),
       keepAtomsCount(this, 0),
       purgeAtomsDeferred(this, 0),
-      tenuredStrings(this, 0),
-      allocNurseryStrings(this, true),
-      tenuredBigInts(this, 0),
-      allocNurseryBigInts(this, true),
       propertyTree_(this, this),
       baseShapes_(this, this),
       initialShapes_(this, this),
       nurseryShapes_(this),
-      data(this, nullptr),
-      isSystem(this, false),
-#ifdef DEBUG
-      gcSweepGroupIndex(0),
-#endif
       finalizationRecordMap_(this, this),
       jitZone_(this, nullptr),
       gcScheduled_(false),
@@ -182,6 +179,7 @@ JS::Zone::Zone(JSRuntime* rt)
   /* Ensure that there are no vtables to mess us up here. */
   MOZ_ASSERT(reinterpret_cast<JS::shadow::Zone*>(this) ==
              static_cast<JS::shadow::Zone*>(this));
+  MOZ_ASSERT(gcSweepGroupIndex == 0);
 }
 
 Zone::~Zone() {