Reapply descriptor array sharing.

test/mjsunit/regress/regress-convert-transition.js

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-#ifndef V8_PLATFORM_POSIX_H_
-#define V8_PLATFORM_POSIX_H_
+var input = '{ "a1":1, "a2":1, "a3":1, "a4":1, "a5":1, "a6":1, "a7":1,\
+               "a8":1, "a9":1, "a10":1, "a11":1, "a12":1, "a13":1}';
+var a = JSON.parse(input);
+a.a = function() { return 10; };
 
-namespace v8 {
-namespace internal {
+// Force conversion of field to slow mode.
+var b = JSON.parse(input);
+b.a = 10;
 
-// Used by platform implementation files during OS::PostSetUp().
-void POSIXPostSetUp();
-
-} }  // namespace v8::internal
-
-#endif  // V8_PLATFORM_POSIX_H_
+// Add another property to the object that would transition to a.
+var c = JSON.parse(input);
+c.x = 10;
+assertEquals(undefined, c.a);