Revert of Remove default value for number of threads in Isolate::Init. (patchset #1 id:1 of https://codereview.chromium.org/978773002/)

Reason for revert:
build failures.

Original issue's description:
> Remove default value for number of threads in Isolate::Init.
>
> Just fixing a TODO.
>
> R=jochen@chromium.org
>
> Committed: https://crrev.com/fbcebbc5bdfd8eed82d121faf07b536eb3142fb7
> Cr-Commit-Position: refs/heads/master@{#26987}

TBR=jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/964743003

Cr-Commit-Position: refs/heads/master@{#26988}