Update components/pdf_strings_grdp/OWNERS.

Point it at pdf/OWNERS.

Change-Id: I8c2dcc590b16c614170214566e72fa99e29d33c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2223459Reviewed-by: Raymes Khoury <raymes@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773887}

components/pdf_strings_grdp/OWNERS

-raymes@chromium.org
+file://pdf/OWNERS
+
 # COMPONENT: Internals>Plugins>PDF