All worker threads should be joined before main thread's V8 isolate is destroyed
All worker threads should be joined before main thread's V8 isolate is destroyed

Currently the following scenario can happen:

1) The main thread calls blink::shutdown().
2) The main thread calls V8Initializer::shutdown(), which destroys the V8 isolate.
3) Worker threads are still running and can trigger Oilpan's GC (<== this accesses the isolate and causes crash).
4) The main thread calls modulesInitializer().shutdown() and joins all workers.

To address the issue, this CL moves 4) to between 1) and 2).

BUG=610987

Review-Url: https://codereview.chromium.org/1969673004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392913}

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