v8/snapshot_toolchain.gni moved

https://chromium-review.googlesource.com/c/v8/v8/+/1612903

BUILD.gn

@@ -8,7 +8,7 @@ import("//tools/generate_library_loader/generate_library_loader.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 import("//tools/v8_context_snapshot/v8_context_snapshot.gni")
-import("//v8/snapshot_toolchain.gni")
+import("//v8/gni/snapshot_toolchain.gni")
 import("build/asar.gni")
 import("build/js_wrap.gni")
 import("build/npm.gni")