Add GN targets for third_party/libwebm

BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 27 matching lines @@
     "//net",
     #"//sdch",
     "//skia",
     #"//third_party/WebKit/Source/platform",
     "//third_party/brotli",
     "//third_party/iccjpeg",
     "//third_party/icu",
     "//third_party/leveldatabase",
     "//third_party/libpng",
     "//third_party/libusb",
+    "//third_party/libwebm",
     "//third_party/libwebp",
     "//third_party/ots",
     "//third_party/qcms",
     "//third_party/re2",
     "//third_party/smhasher:cityhash",
     "//third_party/smhasher:murmurhash3",
     "//third_party/smhasher:pmurhash",
     "//third_party/WebKit/Source/wtf",
     "//third_party/zlib",
     "//third_party:jpeg",
@@ skipping 21 matching lines @@
       "//skia",  # Needs some ARM stuff
       "//third_party/libusb",
       "//third_party/WebKit/Source/wtf",  # TODO(brettw) re-enable for Android.
       "//tools/gn",
       "//ui/base:ui_base",
       "//ui/events",
       "//ui/gfx",
     ]
   }
 }