[gn] Switch custom snapsot bot to gn

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
@@ skipping 10 matching lines @@
 declare_args() {
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = false
 
   # Enable compiler warnings when using V8_DEPRECATED apis.
   v8_deprecation_warnings = false
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
   v8_imminent_deprecation_warnings = false
 
+  # Embeds the given script into the snapshot.
+  v8_embed_script = ""
+
   # Sets -dENABLE_DISASSEMBLER.
   v8_enable_disassembler = false
 
   # Sets -dENABLE_GDB_JIT_INTERFACE.
   v8_enable_gdbjit = false
 
   # Sets -dENABLE_HANDLE_ZAPPING.
   v8_enable_handle_zapping = true
 
   # Enable ECMAScript Internationalization API. Enabling this feature will
@@ skipping 540 matching lines @@
 
 action("run_mksnapshot") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   deps = [
     ":mksnapshot($v8_snapshot_toolchain)",
   ]
 
   script = "tools/run.py"
 
+  sources = []
+
   outputs = [
     "$target_gen_dir/snapshot.cc",
   ]
 
   args = [
     "./" + rebase_path(get_label_info(":mksnapshot($v8_snapshot_toolchain)",
                                       "root_out_dir") + "/mksnapshot",
                        root_build_dir),
     "--startup_src",
     rebase_path("$target_gen_dir/snapshot.cc", root_build_dir),
   ]
 
   if (v8_random_seed != "0") {
     args += [
       "--random-seed",
       v8_random_seed,
     ]
   }
 
   if (v8_use_external_startup_data) {
     outputs += [ "$root_out_dir/snapshot_blob.bin" ]
     args += [
       "--startup_blob",
       rebase_path("$root_out_dir/snapshot_blob.bin", root_build_dir),
     ]
   }
+
+  if (v8_embed_script != "") {
+    sources += [ v8_embed_script ]
+    args += [ rebase_path(v8_embed_script, root_build_dir) ]
+  }
 }
 
 ###############################################################################
 # Source Sets (aka static libraries)
 #
 
 source_set("v8_maybe_snapshot") {
   if (v8_use_snapshot && v8_use_external_startup_data) {
     public_deps = [
       ":v8_external_snapshot",
@@ skipping 1719 matching lines @@
 
   deps = [
     ":fuzzer_support",
   ]
 
   configs = [ ":internal_config" ]
 }
 
 v8_fuzzer("wasm_asmjs_fuzzer") {
 }