Switch libvpx from static_library to none

Follow in the vein of BUILD.gn switch to group()
https://codereview.chromium.org/1364033003

R=blundell@chromium.org
BUG=534943

Review URL: https://codereview.chromium.org/1365973002 .
diff --git a/libvpx.gyp b/libvpx.gyp
index 725d9a9..5ee612e 100644
--- a/libvpx.gyp
+++ b/libvpx.gyp
@@ -7,15 +7,13 @@
       # This libvpx target contains both encoder and decoder.
       # Encoder is configured to be realtime only.
       'target_name': 'libvpx',
-      'type': 'static_library',
+      'type': 'none',
       'dependencies': [
         '<(DEPTH)/third_party/libvpx_new/libvpx.gyp:libvpx_new'
       ],
-      'direct_dependent_settings': {
-        'include_dirs': [
-          '<(DEPTH)/third_party/libvpx_new/source/libvpx',
-        ],
-      },
+      'export_dependent_settings': [
+        '<(DEPTH)/third_party/libvpx_new/libvpx.gyp:libvpx_new'
+      ],
     },
   ],
 }