Roll libvpx 280676:282874

https://codereview.chromium.org/390713002

Fixes issues from previous failed roll which included

https://codereview.chromium.org/375983002
https://codereview.chromium.org/376873009

R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/387003005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282879 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -42,7 +42,7 @@ vars = {
   "google_toolbox_for_mac_revision": "662",
   "libaddressinput_revision": "305",
   "libphonenumber_revision": "621",
-  "libvpx_revision": "280676",
+  "libvpx_revision": "282874",
   "lss_revision": "26",
   "sfntly_revision": "239",
   "lighttpd_revision": "33737",