libwebm,README.chromium: update version & CPEPrefix

libwebm was updated past 1.0.0.28 in:
  http://crrev.com/5b728dd6b14ad4053f79369a503c6530fcb012fc

Change-Id: I52b9d3d10f4aba8df9c0ad70208cd0a0427997fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3064079
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Tom Finegan <tomfinegan@chromium.org>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#911485}
diff --git a/third_party/libwebm/README.chromium b/third_party/libwebm/README.chromium
index 6a3fcd52..cdbc21d6 100644
--- a/third_party/libwebm/README.chromium
+++ b/third_party/libwebm/README.chromium
@@ -1,8 +1,8 @@
 Name: WebM container parser and writer.
 Short Name: libwebm
 URL: http://www.webmproject.org/code/
-Version: unknown
-CPEPrefix: cpe:/a:webmproject:libwebm:1.0.0.27
+Version: 1.0.0.28
+CPEPrefix: cpe:/a:webmproject:libwebm:1.0.0.28
 License: BSD
 License File: source/LICENSE.TXT
 Security Critical: yes