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::Instance* Instance::GetInstance() {
  return this;
}

pp::Instance* Instance::GetInstance() {
  return this;
}
