Do more page load/unload checks in EmbedderTest.

- Keep track of pages in a single map when calling LoadPage(). It is
  simpler and performance is not crucial as the number of loaded pages
  is usually very small.
- Verify UnloadPage() is only called for loaded pages.
- Verify there are no loaded pages in TearDown().
- Verify RenderLoadedPage methods are only rendering loaded pages.
- Fix pages that are using FPDF_LoadPage() and FPDF_ClosePage() when
  they should be using LoadPage() and UnloadPage().
- Disallow calling LoadPage() for the same page number repeatedly. No
  caller does this and it makes verification in UnloadPage() harder.

Change-Id: I58878ea8ade21dde28f1bbebd3a3304ce677561d
Reviewed-on: https://pdfium-review.googlesource.com/25550
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

0 files changed, 0 insertions, 0 deletions