Roll libvpx 254786:255744

BUG=350540

Review URL: https://codereview.chromium.org/191883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255815 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -35,7 +35,7 @@ vars = {
   "google_toolbox_for_mac_revision": "626",
   "libaddressinput_revision": "176",
   "libphonenumber_revision": "621",
-  "libvpx_revision": "254786",
+  "libvpx_revision": "255744",
   "lss_revision": "24",
 
   # These two FFmpeg variables must be updated together.  One is used for SVN