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/public/TraceKind.h

@@ -15,6 +15,7 @@ namespace js {
 class BaseShape;
 class LazyScript;
 class ObjectGroup;
+class RegExpShared;
 class Shape;
 class Scope;
 namespace jit {
@@ -58,13 +59,15 @@ enum class TraceKind
     BaseShape = 0x0F,
     JitCode = 0x1F,
     LazyScript = 0x2F,
-    Scope = 0x3F
+    Scope = 0x3F,
+    RegExpShared = 0x4F
 };
 const static uintptr_t OutOfLineTraceKindMask = 0x07;
 static_assert(uintptr_t(JS::TraceKind::BaseShape) & OutOfLineTraceKindMask, "mask bits are set");
 static_assert(uintptr_t(JS::TraceKind::JitCode) & OutOfLineTraceKindMask, "mask bits are set");
 static_assert(uintptr_t(JS::TraceKind::LazyScript) & OutOfLineTraceKindMask, "mask bits are set");
 static_assert(uintptr_t(JS::TraceKind::Scope) & OutOfLineTraceKindMask, "mask bits are set");
+static_assert(uintptr_t(JS::TraceKind::RegExpShared) & OutOfLineTraceKindMask, "mask bits are set");
 
 // When this header is imported inside SpiderMonkey, the class definitions are
 // available and we can query those definitions to find the correct kind
@@ -87,7 +90,8 @@ struct MapTypeToTraceKind {
     D(Script,        JSScript,          true) \
     D(Shape,         js::Shape,         true) \
     D(String,        JSString,          false) \
-    D(Symbol,        JS::Symbol,        false)
+    D(Symbol,        JS::Symbol,        false) \
+    D(RegExpShared,  js::RegExpShared,  true)
 
 // Map from all public types to their trace kind.
 #define JS_EXPAND_DEF(name, type, _) \