[unseasoned-pdf] Migrate RecordDocumentMetrics()

Move to PdfViewPluginBase in preparation for a common implementation of
DocumentLoadComplete().

Meanwhile, remove some straggling thumbnail code in
out_of_process_instance.h.

Bug: 1140629
Change-Id: Ie138cab7a2b02fe688cc142890bb0321c9c39fa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2782252


Reviewed-by: Hui Yingst <nigi@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866180}