[turbofan] Don't lookup private symbols on prototypes.

R=yangguo@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1427583008

Cr-Commit-Position: refs/heads/master@{#31686}

src/compiler/js-native-context-specialization.cc

@@ -449,6 +449,9 @@ bool JSNativeContextSpecialization::ComputePropertyAccessInfo(
       return false;
     }
 
+    // Don't lookup private symbols on the prototype chain.
+    if (name->IsPrivate()) return false;
+
     // Walk up the prototype chain.
     if (!map->prototype()->IsJSObject()) {
       // Perform the implicit ToObject for primitives here.