Upstream: Yes
Reason: Required for GJS
# HG changeset patch
# User Philip Chimento <philip.chimento@gmail.com>
# Date 1527544053 25200
#      Mon May 28 14:47:33 2018 -0700
# Node ID a50e7374d0952b893f843e9d21d13d5b7ec8564c
# Parent  a466172aed4bc2afc21169b749b8068a4b98c93f
Bug 1464912 - Add exposeToActiveJS specialization for JSString. r=sfink

diff --git a/js/public/RootingAPI.h b/js/public/RootingAPI.h
--- a/js/public/RootingAPI.h
+++ b/js/public/RootingAPI.h
@@ -719,6 +719,10 @@ struct BarrierMethods<JSString*>
     static void postBarrier(JSString** vp, JSString* prev, JSString* next) {
         JS::HeapStringPostBarrier(vp, prev, next);
     }
+    static void exposeToJS(JSString* v) {
+        if (v)
+            js::gc::ExposeGCThingToActiveJS(JS::GCCellPtr(v));
+    }
 };
 
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