[unseasoned-pdf] Get rid of PDFEngine::Client::GetPluginInstance().

Remove the last PPAPI reference in PDFEngine::Client. In its place, add
PDFEngine::Client::SetLastPluginInstance(). Then PDFiumEngine can just
ask the client to do the work without knowing anything about
pp::Instance.

Bug: 1113919,1114263
Change-Id: I4231b8cafad0f4e096a98560fdb360907ca766dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2977438


Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#894866}