Land chromium-side work to clean up handling of v8_target_cpu in the GN build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 76b06bbdc2d1921fc1e593cc269e088ed947b6b0..fe00ba966e8cd7517638a2189ed7d0caf298b0dc 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -16,6 +16,7 @@ import("//build_overrides/v8.gni")
 import("//media/media_options.gni")
 import("//third_party/openh264/openh264_args.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
+import("//v8/snapshot_toolchain.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
@@ -976,7 +977,7 @@ if (!is_ios && !is_android && !is_chromecast) {
     if (!is_chromeos) {
       deps += [
         "//third_party/pdfium/samples:pdfium_test",
-        "//v8:v8_shell($host_toolchain)",
+        "//v8:v8_shell($v8_snapshot_toolchain)",
       ]
     }
     if (is_clang) {