Roll src/third_party/libvpx/source/libvpx/ 91f87e751..4d4231352 (50 commits).

DEPS

 # This file is used to manage the dependencies of the Chromium src repo. It is
 # used by gclient to determine what version of each dependency to check out, and
 # where.
 #
 # For more information, please refer to the official documentation:
 #   https://sites.google.com/a/chromium.org/dev/developers/how-tos/get-the-code
 #
 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 #
@@ skipping 184 matching lines @@
   'src/third_party/bidichecker':
     Var('chromium_git') + '/external/bidichecker/lib.git' + '@' + '97f2aa645b74c28c57eca56992235c79850fa9e0',
 
   'src/third_party/webgl/src':
     Var('chromium_git') + '/external/khronosgroup/webgl.git' + '@' + 'd0783b85bd445f700576dba25bf452d58c112d33',
 
   'src/third_party/webdriver/pylib':
     Var('chromium_git') + '/external/selenium/py.git' + '@' + '5fd78261a75fe08d27ca4835fb6c5ce4b42275bd',
 
   'src/third_party/libvpx/source/libvpx':
-    Var('chromium_git') + '/webm/libvpx.git' + '@' +  '91f87e75135cc9722ee72daf5154424f628a906e',
+    Var('chromium_git') + '/webm/libvpx.git' + '@' +  '4d4231352c8cefdae2e76b7bad4286ec21747c89',
 
   'src/third_party/ffmpeg':
     Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '3f3ad2ea90df08f5907bd997e1ce22e1c19ce215',
 
   'src/third_party/usrsctp/usrsctplib':
     Var('chromium_git') + '/external/github.com/sctplab/usrsctp' + '@' + '8679f2b0bf063ac894dc473debefd61dbbebf622',
 
   'src/third_party/libsrtp':
     Var('chromium_git') + '/chromium/deps/libsrtp.git' + '@' + '0e0936f3013fe5884eac82f95e370c8d460a179f',
 
@@ skipping 909 matching lines @@
 recursedeps = [
   # buildtools provides clang_format, libc++, and libc++abi
   'src/buildtools',
   # android_tools manages the NDK.
   'src/third_party/android_tools',
   # ANGLE manages DEPS that it also owns the build files for, such as dEQP.
   ("src/third_party/angle", "DEPS.chromium"),
   # SwiftShader manages DEPS that it also owns the build files for, such as Subzero.
   ("src/third_party/swiftshader", "DEPS"),
 ]