With this CL, {CreateDefaultPlatform} returns a unique_ptr to indicate
that the caller owns the returned memory. We had several memory leaks
where the memory of the DefaultPlatform did not get deallocated.

In addition, the {TracingController} of the {DefaultPlatform} also gets
received as a unique_ptr. Thereby we document that the {DefaultPlatform}
takes ownership of the {TracingController}. Note that the memory of the
{TracingController} was already owned by the {DefaultPlatform}, but it
was not documented in the interface, and it was used incorrectly in
tests.

This CL fixes the asan issues in 
https://chromium-review.googlesource.com/c/v8/v8/+/753583	
([platform] Implement TaskRunners in the DefaultPlatform)

R=rmcilroy@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I0d1a6d3b22bb8289dc050b1977e4f58381cec675
Reviewed-on: https://chromium-review.googlesource.com/755033Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49349}