Fix v8_current_cpu setting for CrOS builds after r410853.

In r410853 we changed how the GN toolchains move arguments around,
but we missed one reference to the v8_toolchain_cpu in the CrOS
toolchain definitions. This CL fixes that.

TBR=brettw@chromium.org, stevenjb@chromium.org
BUG=634446

Review-Url: https://codereview.chromium.org/2231643002
Cr-Original-Commit-Position: refs/heads/master@{#410933}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 72d6d06aef7e5284e20e30c685ec35557d9e38a7

toolchain/cros/BUILD.gn

@@ -122,7 +122,7 @@ gcc_toolchain("v8_snapshot") {
     } else {
       current_cpu = "x64"
     }
-    v8_toolchain_cpu = v8_target_cpu
+    v8_current_cpu = v8_target_cpu
     current_os = "linux"
     use_sysroot = false
   }