Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error: 'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625

Review URL: https://codereview.chromium.org/2149823003 .

BUILD.gn

@@ -87,12 +87,6 @@ source_set("libyuv") {
     defines += [ "HAVE_JPEG" ]
   }
 
-  if (is_msan) {
-    # MemorySanitizer does not support assembly code yet.
-    # http://crbug.com/344505
-    defines += [ "LIBYUV_DISABLE_X86" ]
-  }
-
   deps = [
     "//third_party:jpeg",
   ]

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1607
+Version: 1608
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1607
+#define LIBYUV_VERSION 1608
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT