[v8 platform] Use modern v8 worker threads API in gin.

Follow-up to v8 CLs:
 - https://chromium-review.googlesource.com/c/v8/v8/+/941946
 - https://chromium-review.googlesource.com/c/v8/v8/+/941442
 - https://chromium-review.googlesource.com/c/v8/v8/+/941944
 - https://chromium-review.googlesource.com/c/v8/v8/+/1033584
 - https://chromium-review.googlesource.com/c/v8/v8/+/978443

This will make Platform::CallBlockingTaskOnWorkerThread() actually
result in a high-priority task which is expected to help blocking
parallel GC tasks get ahead of other async work.

And allow removal of deprecated "background threads" APIs in
v8::Platform.

Bug: 817421
Change-Id: Idb7f369538c23c9b065d90cc5ed48300a5c9724c
Reviewed-on: https://chromium-review.googlesource.com/957761
Reviewed-by: Jeremy Roman <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#556103}
6 files changed