Bug 645417, part 10 - Well-known symbols. r=terrence,r=efaust.

At present there is only one, Symbol.iterator, and it is not hooked up to
anything (this happens in bug 918828). ES6 defines 8 well-known symbols. Each
one is attached to a feature, so we'll add the symbols as we add features.
Symbol.create will appear when @@create semantics are implemented.

js/src/jsatom.cpp

@@ -126,6 +126,7 @@ JSRuntime::initializeAtoms(JSContext *cx)
         commonNames = parentRuntime->commonNames;
         emptyString = parentRuntime->emptyString;
         permanentAtoms = parentRuntime->permanentAtoms;
+        wellKnownSymbols = parentRuntime->wellKnownSymbols;
         return true;
     }
 
@@ -160,30 +161,43 @@ JSRuntime::initializeAtoms(JSContext *cx)
     JS_ASSERT(uintptr_t(names) == uintptr_t(commonNames + 1));
 
     emptyString = commonNames->empty;
+
+    // Create the well-known symbols.
+    wellKnownSymbols = cx->new_<WellKnownSymbols>();
+    if (!wellKnownSymbols)
+        return false;
+
+    ImmutablePropertyNamePtr *descriptions = &commonNames->Symbol_iterator;
+    ImmutableSymbolPtr *symbols = reinterpret_cast<ImmutableSymbolPtr *>(wellKnownSymbols);
+    for (size_t i = 0; i < JS::WellKnownSymbolLimit; i++) {
+        JS::Symbol *symbol = JS::Symbol::new_(cx, JS::SymbolCode(i), descriptions[i]);
+        if (!symbol) {
+            js_ReportOutOfMemory(cx);
+            return false;
+        }
+        symbols[i].init(symbol);
+    }
+
     return true;
 }
 
 void
 JSRuntime::finishAtoms()
 {
-    if (atoms_)
-        js_delete(atoms_);
+    js_delete(atoms_);
 
     if (!parentRuntime) {
-        if (staticStrings)
-            js_delete(staticStrings);
-
-        if (commonNames)
-            js_delete(commonNames);
-
-        if (permanentAtoms)
-            js_delete(permanentAtoms);
+        js_delete(staticStrings);
+        js_delete(commonNames);
+        js_delete(permanentAtoms);
+        js_delete(wellKnownSymbols);
     }
 
     atoms_ = nullptr;
     staticStrings = nullptr;
     commonNames = nullptr;
     permanentAtoms = nullptr;
+    wellKnownSymbols = nullptr;
     emptyString = nullptr;
 }
 
@@ -227,6 +241,20 @@ js::MarkPermanentAtoms(JSTracer *trc)
     }
 }
 
+void
+js::MarkWellKnownSymbols(JSTracer *trc)
+{
+    JSRuntime *rt = trc->runtime();
+
+    if (rt->parentRuntime)
+        return;
+
+    if (WellKnownSymbols *wks = rt->wellKnownSymbols) {
+        for (size_t i = 0; i < JS::WellKnownSymbolLimit; i++)
+            MarkWellKnownSymbol(trc, wks->get(i));
+    }
+}
+
 void
 JSRuntime::sweepAtoms()
 {