This function is always called right after creating the DefaultPlatform,
hence merge it into the constructor.

R=mlippautz@chromium.org

Change-Id: I4afb14c83740224056157665db6b854c659da0c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182635Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67573}