[cleanup] Remove last uses of CreateDefaultPlatform

Convert all remaining v8::platform::CreateDefaultPlatform uses to
v8::platform::NewDefaultPlatform which returns a std::unique_ptr. This
method was originally deprecated in
https://chromium-review.googlesource.com/755033.

Bug: v8:8562
Change-Id: Ia3dce828f5cc9c757728c2c135398699f8fe2c50
Reviewed-on: https://chromium-review.googlesource.com/c/1388466
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618544}
3 files changed