Grit: Add WebUI OWNERs for resource_ids.spec

Bug: 1132403
Change-Id: Iab13dc6afca566f5056f37a25849ac907f0efb2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466336Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816240}

tools/gritsettings/OWNERS

 file://tools/grit/OWNERS
 
 huangs@chromium.org
+
+# For WebUI related .grd files.
+per-file resource_ids.spec=file://ui/webui/PLATFORM_OWNERS