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}
void Instance::GetDocumentPassword(
    pp::CompletionCallbackWithOutput<pp::Var> callback) {
  std::string message(GetLocalizedString(PP_RESOURCESTRING_PDFGETPASSWORD));
  pp::Var result = pp::PDF::ModalPromptForPassword(this, message);
  *callback.output() = result.pp_var();
  callback.Run(PP_OK);
}

void Instance::GetDocumentPassword(
    pp::CompletionCallbackWithOutput<pp::Var> callback) {
  std::string message(GetLocalizedString(PP_RESOURCESTRING_PDFGETPASSWORD));
  pp::Var result = pp::PDF::ModalPromptForPassword(this, message);
  *callback.output() = result.pp_var();
  callback.Run(PP_OK);
}
