Remove support for specifying the number of available threads

Remove support for specifying the number of available threads

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}