PDF Viewer: Add some WebUI owners for pdf_extension_test.*

This file frequently needs to be modified for JS/front end only changes
because:

- IN_PROC_BROWSER_TEST_F calls are located there, rather than in a
  separate JS file as is the case for other Web UIs.
- Several tests in this file work by executing inline JS strings, rather
  than executing the contents of a separate JS file

As a result, adding some Web UI OWNERs for this file, so that a separate
reviewer does not have to be added for these types of changes.

Change-Id: I05ae96abc55a87a6d9f820d47d58aba50cde4882
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274178Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783663}

chrome/browser/pdf/OWNERS

 file://pdf/OWNERS
+
+per-file pdf_extension_test.*=dpapad@chromium.org
+per-file pdf_extension_test.*=rbpotter@chromium.org
 # COMPONENT: Internals>Plugins>PDF