This enables side-by-side snapshots with and without untrusted-code
mitigations. It'll be the default in all V8 stand-alone builds
with external startup data. Internal snapshots are not supported.

The files snapshot_blob.bin and snapshot_blob_trusted.bin will be
bundled with V8 on swarming and the correct file is loaded dependent
on the --untrusted-code-mitigations runtime flag.

Likewise we embed two snapshots for builtins.

Side-by-side snapshots won't be supported in Chromium.

Bug: v8:7441
Change-Id: I2949ddfd5773649946b1c8e74751d48ad1d9c524
Reviewed-on: https://chromium-review.googlesource.com/960004
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52028}