IsEnabled() is checked before and after taking the lock. Move the first
check into NewMessageBuilder() to make this more obvious to the reader.

Bug: v8:10315
Change-Id: Iee1000a209f3ae7d07884f1cbb4e0daf43a58a9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2414227Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69968}