Enable Instruction Scheduler for snapshots.

Bug: 
Change-Id: I41ce841cd20bab26de29437d4365f011c98ad53e
Reviewed-on: https://chromium-review.googlesource.com/739392Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#49006}

BUILD.gn

@@ -792,6 +792,7 @@ action("run_mksnapshot") {
     "./" + rebase_path(get_label_info(":mksnapshot($v8_snapshot_toolchain)",
                                       "root_out_dir") + "/mksnapshot",
                        root_build_dir),
+    "--turbo_instruction_scheduling",
     "--startup_src",
     rebase_path("$target_gen_dir/snapshot.cc", root_build_dir),
   ]