PDF Viewer: Add some WebUI OWNERS for pdf_extension_util.*

This file contains UI strings and flags, which frequently need to be
updated for front end PDF Viewer changes. Add a few Web UI OWNERS for
this file, so a separate reviewer does not need to be added for it.

Change-Id: I9cf46bb7d001c146d9fb47efbf1f27f7bd94a602
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274183
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783763}

chrome/browser/pdf/OWNERS

@@ -2,4 +2,6 @@ file://pdf/OWNERS
 
 per-file pdf_extension_test.*=dpapad@chromium.org
 per-file pdf_extension_test.*=rbpotter@chromium.org
+per-file pdf_extension_util.*=dpapad@chromium.org
+per-file pdf_extension_util.*=rbpotter@chromium.org
 # COMPONENT: Internals>Plugins>PDF