Issue #2083 - Part 1: Make RegExpShared a GC thing.

Based on Mozilla bug 1345177.
Changes from the original bug's patch:

- We don't have JS::CurrentThreadIsHeapCollecting, so let's use
  trc->runtime()->isHeapCollecting() instead.
- Mozilla bug 1337117 renamed runtimeFromMainThread to
  runtimeFromActiveCooperatingThread for Firefox 54, so let's use the former

js/xpconnect/src/XPCJSContext.cpp

@@ -1837,6 +1837,14 @@ ReportZoneStats(const JS::ZoneStats& zStats,
         zStats.scopesMallocHeap,
         "Arrays of binding names and other binding-related data.");
 
+    ZCREPORT_GC_BYTES(pathPrefix + NS_LITERAL_CSTRING("regexp-shareds/gc-heap"),
+        zStats.regExpSharedsGCHeap,
+        "Shared compiled regexp data.");
+
+    ZCREPORT_BYTES(pathPrefix + NS_LITERAL_CSTRING("regexp-shareds/malloc-heap"),
+        zStats.regExpSharedsMallocHeap,
+        "Shared compiled regexp data.");
+
     ZCREPORT_BYTES(pathPrefix + NS_LITERAL_CSTRING("type-pool"),
         zStats.typePool,
         "Type sets and related data.");
@@ -2855,6 +2863,10 @@ JSReporter::CollectReports(WindowPaths* windowPaths,
         KIND_OTHER, rtStats.zTotals.unusedGCThings.jitcode,
         "Unused jitcode cells within non-empty arenas.");
 
+    REPORT_BYTES(NS_LITERAL_CSTRING("js-main-runtime-gc-heap-committed/unused/gc-things/regexp-shareds"),
+        KIND_OTHER, rtStats.zTotals.unusedGCThings.regExpShared,
+        "Unused regexpshared cells within non-empty arenas.");
+
     REPORT_BYTES(NS_LITERAL_CSTRING("js-main-runtime-gc-heap-committed/used/chunk-admin"),
         KIND_OTHER, rtStats.gcHeapChunkAdmin,
         "The same as 'explicit/js-non-window/gc-heap/chunk-admin'.");
@@ -2906,6 +2918,10 @@ JSReporter::CollectReports(WindowPaths* windowPaths,
         KIND_OTHER, rtStats.zTotals.jitCodesGCHeap,
         "Used jitcode cells.");
 
+    MREPORT_BYTES(NS_LITERAL_CSTRING("js-main-runtime-gc-heap-committed/used/gc-things/regexp-shareds"),
+        KIND_OTHER, rtStats.zTotals.regExpSharedsGCHeap,
+        "Used regexpshared cells.");
+
     MOZ_ASSERT(gcThingTotal == rtStats.gcHeapGCThings);
 
     // Report xpconnect.