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Instead of passing a using base::ThreadTaskRunnerHandle::Get(), which can
fail if it's not set up yet, pass nullptr, which causes it to just run on
a background thread. This is fine for the shared memory API as it is not
tied to any particular Isolate.

Additionally this moves registration into the V8SharedMemoryDumpProvider
constructor which itself is created as a Singleton after the first V8
Isolate is created. This ensures that the shared memory is actually set up
before the first dump is provided.

Bug: v8:7464, 1023644
Change-Id: I18fde2c388c3b4c01191b00b4ec441d6964b473b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912205
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715661}