Roll libyuv from 1703 to 1708

Contains the following changes:
a9626b9 Disable AVX512 for iOS simulator xcode 9 builds.
a694e33 Make the gflags library optional in the CMake build
d4cfc7b Fix building with clang targeting x86_32 mingw
efb70e5 Fix building the library with cmake for windows
816b7b1 Add __attribute__ ((__target__ ("avx512vbmi")))
4ad3334 Pass float parameters via vector 2 float and "w" for scalar multiply. by Frank Barchard
d868089 Fix building with clang targeting x86_32 mingw by Martin Storsjö
548ec65 Require clang 6 for AVX512 support by Frank Barchard
c21af29 jpeg remove empty function declarations by Frank Barchard
fdad629 Add a method to force the CPU flags by Jay Civelli
6ecd76a Make libyuv use the hermetic xcode toolchain. by Patrik Höglund
9d70f13 cpuid sandbox friendlier avoiding getenv() by Frank Barchard
83aa751 AVX512 VMBI version of ARGBToRGB24 by Frank Barchard
004954c cpu disables for AVX 512 and unittest show decimal by Frank Barchard
1d509f2 ARGBToRGB24_AVX2 version by Frank Barchard
3009890 NV21ToRGB24_AVX2 and SSSE3 by Frank Barchard

Change-Id: I5dcd923114ac6a696408ca6e5206f11aabb232fc
Reviewed-on: https://chromium-review.googlesource.com/988217Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547935}

DEPS

@@ -670,7 +670,7 @@ deps = {
     Var('chromium_git') + '/webm/libwebm.git' + '@' + 'b03c65468b06d097f27235d93d76bfc45f490ede',
 
   'src/third_party/libyuv':
-    Var('chromium_git') + '/libyuv/libyuv.git' + '@' + '98a0a157dcf5dee0882b2dfcc9578ab1f44afb12',  # from r1703
+    Var('chromium_git') + '/libyuv/libyuv.git' + '@' + 'a9626b9daf62a9b260737e9c2de821ad087b19a1',  # from r1708
 
   'src/third_party/lighttpd': {
       'url': Var('chromium_git') + '/chromium/deps/lighttpd.git' + '@' + Var('lighttpd_revision'),