Roll libwebm 8be639:0d4cb4

This roll:
- Allows libwebm to build using clang-cl.
- Allows removal of a workaround from libwebm's BUILD.gn.

R=thakis

Review URL: https://codereview.chromium.org/403063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284284 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -317,7 +317,7 @@ deps = {
 
   "src/third_party/libwebm/source":
     Var("chromium_git") +
-      "/webm/libwebm.git@8be63972fdd7ae8c041778f8ba674ade903395c9",
+      "/webm/libwebm.git@0d4cb404ea4195e5e21d04db2c955615535ce62e",
 
   "src/third_party/openssl":
     "/trunk/deps/third_party/openssl@" + Var("openssl_revision"),