Update V8 to r5091 as required by WebKit r63859.

Change-Id: I8e35d765e6f6c7f89eccff900e1cabe2d5dd6110
diff --git a/test/mjsunit/json.js b/test/mjsunit/json.js
index 85457cd..945b662 100644
--- a/test/mjsunit/json.js
+++ b/test/mjsunit/json.js
@@ -85,7 +85,7 @@
 };
 assertEquals(null, n4.toJSON());
 
-assertEquals(Object.prototype, JSON.__proto__);
+assertTrue(Object.prototype === JSON.__proto__);
 assertEquals("[object JSON]", Object.prototype.toString.call(JSON));
 
 // DontEnum
@@ -313,3 +313,7 @@
 var x = 0;
 eval("(1); x++; (1)");
 TestInvalid('1); x++; (1');
+
+// Test string conversion of argument.
+var o = { toString: function() { return "42"; } };
+assertEquals(42, JSON.parse(o));