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Currently, the main thread WebThread is owned by the embedder. We'd
like to shift the ownership to Platform, so that we can stop exposing
WebThread to embedders.

As a first step, this patch introduces a new initialization method
Platform::CreateMainThreadAndInitialize(), which is parallel to
Platform::Initialize(). It is targeted to tests or other use cases
which only require a simple execution environment of Blink. Currently,
each embedder implements its own WebThread, but I'd like to unify
those and remove them from non-Blink code eventually.

The new CreateMainThreadAndInitialize() lets Platform own the main
thread WebThread object, unlike Initialize(). Currently, Platform
needs to handle two different cases: main thread owned or not owned,
but this is only temporary, and they will eventually converge to the
"owned" case.

As a test bed, v8_context_snapshot_generator is migrated to the
new initialization method.

Bug: 826203
Change-Id: I14436509f420a5d73f2ce64722f41da5756fa45b
Reviewed-on: https://chromium-review.googlesource.com/1163560
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581136}