Add BUILD.gn for third_party/yasm.

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 59 matching lines @@
     "//third_party/mesa",
     "//third_party/ots",
     "//third_party/protobuf:protobuf_lite",
     "//third_party/qcms",
     "//third_party/re2",
     "//third_party/smhasher:cityhash",
     "//third_party/smhasher:murmurhash3",
     "//third_party/smhasher:pmurhash",
     # "//third_party/WebKit/Source/platform",
     "//third_party/WebKit/Source/wtf",
+    "//third_party/yasm($host_toolchain)",
     "//third_party/zlib",
     "//third_party:jpeg",
     "//tools/gn",
     "//ui/aura",
     "//ui/accessibility",
     "//ui/base",
     "//ui/events",
     "//ui/gfx",
     "//ui/gl",
     "//ui/native_theme",
@@ skipping 42 matching lines @@
       "//third_party/WebKit/Source/wtf",  # TODO(brettw) re-enable for Android.
       "//tools/gn",
       "//ui/aura",
 
       # Not tested on Android yet:
       "//webkit/browser:storage",
       "//webkit/child",
     ]
   }
 }