Let PDFium handle event when there is not yet a visible page.

Speculative fix for 415307. CF will confirm.
The stack trace for that bug indicates an attempt to index by -1, which is consistent with no visible page.

BUG=415307

Review URL: https://codereview.chromium.org/560133004

Cr-Commit-Position: refs/heads/master@{#295421}
pp::Var Instance::GetInstanceObject() {
  if (instance_object_.is_undefined()) {
    PDFScriptableObject* object = new PDFScriptableObject(this);
    instance_object_ = pp::VarPrivate(this, object);
  }

  return instance_object_;
}

pp::Var Instance::GetInstanceObject() {
  if (instance_object_.is_undefined()) {
    PDFScriptableObject* object = new PDFScriptableObject(this);
    instance_object_ = pp::VarPrivate(this, object);
  }

  return instance_object_;
}
