Update OWNERS files for various pdf directories.

- Remove dsinclair@, who is inactive.
- Fill chrome/browser/resources/pdf/OWNERS, which would be empty, with
  folks who have made significant changes there.

Change-Id: I80f99658eb21085579eb3783a335f11cd83431a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3125464
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919914}

chrome/browser/resources/pdf/OWNERS

@@ -1 +1,3 @@
-dsinclair@chromium.org
+dhoss@chromium.org
+dpapad@chromium.org
+rbpotter@chromium.org

pdf/OWNERS

@@ -1,6 +1,5 @@
 set noparent
 dhoss@chromium.org
-dsinclair@chromium.org
 kmoon@chromium.org
 thestig@chromium.org