Generate the thumbnail onto the SkBitmap of a Thumbnail object using
PDFium's FPDF_RenderPageBitmap().

Add a test PDF and expectation PNGs of its thumbnails at different
resolutions. Compare the rendered thumbnails to the expectation PNGs.

Bug: 652400
Change-Id: Id4c5e7f7a93822c6c38efdd63657e3e8002cd9cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382687Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805961}