[unseasoned-pdf] Migrate HandleSetReadOnlyMessage()

Migrate from a Pepper OutOfProcessInstance implementation to a common
PdfViewPluginBase one.

Bug: 1109796
Change-Id: I8f5a677cc3090f808e2c1c57f2ba0158aa0a836f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2660116


Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Hui Yingst <nigi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848820}