Roll libwebm fb6b6e:acf788

BUG=321825
R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/293053015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272274 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -318,7 +318,7 @@ deps = {
 
   "src/third_party/libwebm/source":
     Var("chromium_git") +
-      "/webm/libwebm.git@fb6b6e64444c637f27d103fd113e0c7bf4f107dd",
+      "/webm/libwebm.git@acf788bedd1ddc0f4a8553c28a8b4e2266accbc9",
 
   "src/third_party/openssl":
     "/trunk/deps/third_party/openssl@" + Var("openssl_revision"),