diff --git a/mozilla/nsprpub/lib/ds/plarena.c b/mozilla/nsprpub/lib/ds/plarena.c
index 5b1e53af1f9..cc2ffb00a89 100644
--- a/mozilla/nsprpub/lib/ds/plarena.c
+++ b/mozilla/nsprpub/lib/ds/plarena.c
@@ -345,6 +345,22 @@ PR_IMPLEMENT(void) PL_ArenaFinish(void)
     once = pristineCallOnce;
 }
 
+PR_IMPLEMENT(size_t) PL_SizeOfArenaPoolExcludingPool(
+    PLArenaPool *pool, PLMallocSizeFn mallocSizeOf)
+{
+    /*
+     * The first PLArena is within |pool|, so don't measure it.  Subsequent
+     * PLArenas are separate and must measured.
+     */
+    size_t size = 0;
+    PLArena *arena = pool->first.next;
+    while (arena) {
+        size += mallocSizeOf(arena);
+        arena = arena->next;
+    }
+    return size;
+}
+
 #ifdef PL_ARENAMETER
 PR_IMPLEMENT(void) PL_ArenaCountAllocation(PLArenaPool *pool, PRUint32 nb)
 {
diff --git a/mozilla/nsprpub/lib/ds/plarenas.h b/mozilla/nsprpub/lib/ds/plarenas.h
index b4705624318..4cbe32c4c39 100644
--- a/mozilla/nsprpub/lib/ds/plarenas.h
+++ b/mozilla/nsprpub/lib/ds/plarenas.h
@@ -60,6 +60,19 @@ PR_EXTERN(void) PL_ArenaRelease(PLArenaPool *pool, char *mark);
 */
 PR_EXTERN(void) PL_ClearArenaPool(PLArenaPool *pool, PRInt32 pattern);
 
+/*
+** A function like malloc_size() or malloc_usable_size() that returns the
+** size of a heap block.
+*/
+typedef size_t (*PLMallocSizeFn)(const void *ptr);
+
+/*
+** Return all memory used by a PLArenaPool, excluding the PLArenaPool
+** structure.
+*/
+PR_EXTERN(size_t) PL_SizeOfArenaPoolExcludingPool(
+    PLArenaPool *pool, PLMallocSizeFn mallocSizeOf);
+
 PR_END_EXTERN_C
 
 #endif /* PLARENAS_H */
diff --git a/mozilla/nsprpub/lib/ds/plds.def b/mozilla/nsprpub/lib/ds/plds.def
index 5c672abb344..cc54a4de2e7 100644
--- a/mozilla/nsprpub/lib/ds/plds.def
+++ b/mozilla/nsprpub/lib/ds/plds.def
@@ -54,3 +54,7 @@ PL_HashTableRawLookupConst;
 ;+    global:
 PL_ClearArenaPool;
 ;+} NSPR_4.1;
+;+NSPR_4.9.6 {
+;+    global:
+PL_SizeOfArenaPoolExcludingPool;
+;+} NSPR_4.8.5;