Add libvpx wrapper to media to support VP9 video decoding.

DEPS

 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 
 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   "googlecode_url": "http://%s.googlecode.com/svn",
   "sourceforge_url": "http://%(repo)s.svn.sourceforge.net/svnroot/%(repo)s",
   "webkit_trunk": "http://svn.webkit.org/repository/webkit/trunk",
   "nacl_trunk": "http://src.chromium.org/native_client/trunk",
   "webkit_revision": "138240",
   "chromium_git": "https://chromium.googlesource.com",
   "chromiumos_git": "https://chromium.googlesource.com/chromiumos",
   "swig_revision": "69281",
   "nacl_revision": "10473",
   # After changing nacl_revision, run 'glient sync' and check native_client/DEPS
   # to update other nacl_*_revision's.
   "nacl_tools_revision": "10001",  # native_client/DEPS: tools_rev
   "gtm_revision": "578",
 
   "libjingle_revision": "251",
   "libphonenumber_revision": "456",
-  "libvpx_revision": "173187",
+  "libvpx_revision": "174246",
   "lss_revision": "17",
 
   # These two FFmpeg variables must be updated together.  One is used for SVN
   # checkouts and the other for Git checkouts.
   "ffmpeg_revision": "168654",
   "ffmpeg_hash": "82ae69cc47f25f9a59cf2ca8742e3391c34e7602",
 
   "sfntly_revision": "134",
   "skia_revision": "6910",
   # Three lines of non-changing comments so that
@@ skipping 629 matching lines @@
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
   {
     # Check for landmines (reasons to clobber the build).
     "pattern": ".",
     "action": ["python", "src/build/landmines.py"],
   },
 ]