Don't touch the prototype chain to get the private script controller.

third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp

Index: third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp b/third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp
index 77b88f46afa5d47ffc73ea9549231bf1216244de..08b18f9db1b37e088b0a12b3c360e2c279b9a84a 100644
--- a/third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp
@@ -57,12 +57,12 @@ static v8::Local<v8::Value> compileAndRunPrivateScript(ScriptState* scriptState,
 
   v8::Local<v8::Context> context = scriptState->context();
   v8::Local<v8::Object> global = context->Global();
-  v8::Local<v8::Value> privateScriptController =
-      global->Get(context, v8String(isolate, "privateScriptController"))
-          .ToLocalChecked();
-  RELEASE_ASSERT(privateScriptController->IsUndefined() ||
-                 privateScriptController->IsObject());
-  if (privateScriptController->IsObject()) {
+  v8::Local<v8::String> key = v8String(isolate, "privateScriptController");
+
+  if (global->HasOwnProperty(context, key).FromMaybe(false)) {

[Yuki, 2016/11/28 08:01:29]

Could you use FromJust() instead?

When FromMaybe(defaultValue) returns the defaultValue, then an exception must be
thrown by HasOwnProperty().  We don't want that exception to be unhandled. 
Crash is somewhat better than unhandled exceptions.

Since it's unlikely that HasOwnProperty throws an exception, we don't need to
care too much about the crash.

[haraken, 2016/11/28 08:04:57]

We should use ToChecked() :) The plan is to deprecate all v8Call macros and
confusing V8 APIs like fromMaybe, fromJust etc and replace them with
To/ToLocal/ToChecked/ToLocalChecked consistently.

We should have a better comment about this. I'll add it.

[Mariusz Mlynski, 2016/11/28 08:18:20]

Done.

+    v8::Local<v8::Value> privateScriptController =
+        global->Get(context, key).ToLocalChecked();
+    CHECK(privateScriptController->IsObject());
     v8::Local<v8::Object> privateScriptControllerObject =
         privateScriptController.As<v8::Object>();
     v8::Local<v8::Value> importFunctionValue =