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::Alert(const std::string& message) {
  GetWindowObject().Call("alert", message);
}

void Instance::Alert(const std::string& message) {
  GetWindowObject().Call("alert", message);
}
