GitLab

Handle messages requesting thumbnails from the PDF plugin.
OutOfProcessInstance provides the callbacks for responding to the
requests when the thumbnails are ready and passes the request to the PDF
engine.

Bug: 652400
Change-Id: I2ed5acfa4fcfbb0167a38a6149017756f44c36d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336312
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810854}