Move private classes/structs in PDFiumPage

This CL moves all the private classes and structs to the top of private
section in PDFiumPage.

Bug: 1030242
Change-Id: I7450e69c27d2dcfa909ef32be80158919a60eb20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2257461


Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Mansi Awasthi <maawas@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#780834}