Document the users and OWNERS of //third_party/ply.

It wasn't entirely clear who was using //third_party/ply,
so this CL documents that (PPAPI, Mojo, and Blink) and adds
appropriate OWNERS for the directory.

Bug: 998358

Change-Id: Ieda359712adb7b0882ca5ff443e92305440cb14c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824066Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699864}

third_party/ply/OWNERS

+bbudge@chromium.org
+rockot@google.com
+
+file://tools/idl_parser/OWNERS
+
 # COMPONENT: Tools

third_party/ply/README.chromium

@@ -6,6 +6,9 @@ License File: LICENSE
 Security Critical: no
 Version: 3.11
 
+PLY is used by (at least) the Mojo python bindings, the PPAPI
+IDL generator, and the Blink IDL generator.
+
 This directory contains a copy of these ply-3.11 components:
 
 README            ply-3.11/README.md