The embedder can control how many threads it wants to use via the
v8::Platform implementation. V8 internally doesn't spin up threads
anymore. If the embedder doesn't want to use any threads at all, it's
v8::Platform implementation must either run the background jobs on
the foreground thread, or the embedder should specify --predictable

BUG=none
R=yangguo@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/1064723005

Cr-Commit-Position: refs/heads/master@{#27833}