Bug 1300834. GC slice logging should say what process the slice is for. r=smaug

dom/base/nsJSEnvironment.cpp

@@ -2186,11 +2186,15 @@ DOMGCSliceCallback(JSContext* aCx, JS::GCProgress aProgress, const JS::GCDescrip
       }
 
       if (sPostGCEventsToConsole) {
-        nsString gcstats;
+        NS_NAMED_LITERAL_STRING(kFmt, "[%s] ");
+        nsString prefix, gcstats;
         gcstats.Adopt(aDesc.formatSliceMessage(aCx));
+        prefix.Adopt(nsTextFormatter::smprintf(kFmt.get(),
+                                               ProcessNameForCollectorLog()));
+        nsString msg = prefix + gcstats;
         nsCOMPtr<nsIConsoleService> cs = do_GetService(NS_CONSOLESERVICE_CONTRACTID);
         if (cs) {
-          cs->LogStringMessage(gcstats.get());
+          cs->LogStringMessage(msg.get());
         }
       }