Add CPEPrefix for NSS.

This will enable automatic tooling to raise a security crbug if
a future security vulnerability (CVE) is reported.

Change-Id: I1ace4eb8ba7ef71d884ab0d9f52b3639075dd47c
Bug: 895969
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464069
Auto-Submit: Adrian Taylor <adetaylor@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815952}

net/third_party/nss/README.chromium

@@ -4,6 +4,7 @@ Version: 3.23 RTM
 Security Critical: Yes
 License: MPL 2
 License File: LICENSE
+CPEPrefix: cpe:/a:mozilla:nss:3.23
 
 This directory includes a file derived from NSS's libssl, from the hg repo at:
   https://hg.mozilla.org/projects/nss