Roll libvpx: cc9c0c:5da40c

Cherry-pick potential divide-by-zero fix
https://codereview.chromium.org/877783004

This includes remnants of an attempt to turn on AVX2. That failed and the
critical parts have been reverted. However, the attempt revealed some necessary
changes which have been kept for the next attempt. These constitute a no-op for
this particular roll.

Enable AVX2:
https://codereview.chromium.org/863143002

Fix GN build:
https://codereview.chromium.org/865893004
https://codereview.chromium.org/817293008
https://codereview.chromium.org/866273003

Fix cpu detection:
https://codereview.chromium.org/864393002

Disable AVX2:
https://codereview.chromium.org/880513003

R=tomfinegan@chromium.org
R=yaowu@chromium.org
BUG=396572

Review URL: https://codereview.chromium.org/883503002

Cr-Commit-Position: refs/heads/master@{#313182}

DEPS

@@ -40,7 +40,7 @@ vars = {
   'pdfium_git': 'https://pdfium.googlesource.com',
   'skia_git': 'https://skia.googlesource.com',
   'boringssl_git': 'https://boringssl.googlesource.com',
-  'libvpx_revision': 'cc9c0c5198e416173f480fb1f11e052f5c29a95b',
+  'libvpx_revision': '5da40ca3395cd09443f3d4a3f8b86e495a6df364',
   'sfntly_revision': '1bdaae8fc788a5ac8936d68bf24f37d977a13dac',
   'skia_revision': '1d124c3142acf1a678de9c3f3189bf0c56d32a39',
   # Three lines of non-changing comments so that