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::ConfigureProgressBar() {
  pp::ImageData background;
  std::vector<pp::ImageData> images = GetProgressBarResources(&background);
  progress_bar_.Reconfigure(background, images, device_scale_);
}

void Instance::ConfigureProgressBar() {
  pp::ImageData background;
  std::vector<pp::ImageData> images = GetProgressBarResources(&background);
  progress_bar_.Reconfigure(background, images, device_scale_);
}
