Updating the 'Protected Content Interface for libva' README.chromium to include Revision

Adding Revision field based on 'Version'.

```
$ git remote -v # This redirects to the provided URL.
origin	https://github.com/intel/libva (fetch)
origin	https://github.com/intel/libva (push)

$ git show-ref --tags 2.9.0
87a25a4e6935bccc729a7b0d3bbcb7c8740c4024 refs/tags/2.9.0
```

R=jkardatzke@chromium.org, marcheu@chromium.org

Bug: chromium:350851535
Change-Id: Id7e62ef52ec0118d6d1df4fb667e4cd1e3054d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5682773
Auto-Submit: Jordan Brown <rop@google.com>
Commit-Queue: J Kardatzke <jkardatzke@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: J Kardatzke <jkardatzke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1325597}
diff --git a/third_party/libva_protected_content/README.chromium b/third_party/libva_protected_content/README.chromium
index 8d14f65..ac9a3b6a 100644
--- a/third_party/libva_protected_content/README.chromium
+++ b/third_party/libva_protected_content/README.chromium
@@ -2,6 +2,7 @@
 Short Name: libva_protected_content
 URL: https://github.com/intel/libva
 Version: 2.9.0
+Revision: 87a25a4e6935bccc729a7b0d3bbcb7c8740c4024
 Date: 2020-11-02
 License: MIT
 License File: LICENSE