Copy visible_pages_ when iterating over it.

On this case, a call inside the loop may cause visible_pages_ to
change.

Bug: 822091
Change-Id: I41b0715faa6fe3e39203cd9142cf5ea38e59aefb
Reviewed-on: https://chromium-review.googlesource.com/964592
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543494}
gfx::PointF PDFiumEngine::TransformPagePoint(int page_index,
                                             const gfx::PointF& page_xy) {
  DCHECK(PageIndexInBounds(page_index));
  return pages_[page_index]->TransformPageToScreenXY(page_xy);
}

gfx::PointF PDFiumEngine::TransformPagePoint(int page_index,
                                             const gfx::PointF& page_xy) {
  DCHECK(PageIndexInBounds(page_index));
  return pages_[page_index]->TransformPageToScreenXY(page_xy);
}
