Add CHROMIUM define when building libyuv with GN

- tells cpu_id.cc to avoid sys_call

Bug: None
Change-Id: I61cd87fd130a6106306cf0f0cc3e36028fa5635f
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/5378424
Reviewed-by: richard winterton <rrwinterton@gmail.com>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

BUILD.gn

@@ -29,7 +29,9 @@ config("libyuv_config") {
       ldflags = [ "-Wl,--dynamic-linker,/system/bin/linker64" ]
     }
   }
-  defines = []
+
+  # Define CHROMIUM to tell cpu_id to avoid sandbox unsafe system calls.
+  defines = [ "CHROMIUM" ]
   if (!libyuv_use_neon) {
     defines += [ "LIBYUV_DISABLE_NEON" ]
   }