Add libwebm in third_party.

DEPS

 # This file is automatically processed to create .DEPS.git which is the file
 # that gclient uses under git.
 #
 # See http://code.google.com/p/chromium/wiki/UsingGit
 #
 # To test manually, run:
 #   python tools/deps2git/deps2git.py -o .DEPS.git
 #   gclient runhooks
 # DO NOT CHECK IN CHANGES TO .DEPS.git. It will be automatically updated by
 # a bot when you modify this one.
@@ skipping 275 matching lines @@
 
   "src/third_party/mesa/src":
     "/trunk/deps/third_party/mesa@240938",
 
   "src/third_party/cld_2/src":
     (Var("googlecode_url") % "cld2") + "/trunk@84",
 
   "src/third_party/html_office":
      Var("chromium_git") +
          "/chromium/html-office-public.git@1d04f3dfbd8bd604190dfb6f5a030d30233ba6ea",
+
+  "src/third_party/libwebm/source":
+    Var("chromium_git") +
+      "/webm/libwebm.git@0f7815b036651e242ec8c2fcfb59fe54f69be1a8",
 }
 
 
 deps_os = {
   "win": {
     "src/chrome/tools/test/reference_build/chrome_win":
       "/trunk/deps/reference_builds/chrome_win@237381",
 
     "src/third_party/cygwin":
       "/trunk/deps/third_party/cygwin@231940",
@@ skipping 437 matching lines @@
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
   {
     # Check for landmines (reasons to clobber the build).
     "name": "landmines",
     "pattern": ".",
     "action": ["python", "src/build/landmines.py"],
   },
 ]