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::DestroyHorizontalScrollbar() {
  if (!h_scrollbar_.get())
    return;
  if (h_scrollbar_->GetValue())
    engine_->ScrolledToXPosition(0);
  h_scrollbar_.reset();
}

void Instance::DestroyHorizontalScrollbar() {
  if (!h_scrollbar_.get())
    return;
  if (h_scrollbar_->GetValue())
    engine_->ScrolledToXPosition(0);
  h_scrollbar_.reset();
}
