Both sample are essentially the same up to string constants since
cppgc's default platform started using libplatform.
The only diff between the sample is whether we call
v8::V8::IntializePlatform or cppgc::InitializeProcess.

Drive-by: replace CPPGC_BUILD_IN_V8 with CPPGC_IS_STANDALONE which is
          more descriptive.

Bug: chromium:1056170
Change-Id: I8fdeb59c3345af77f1bccd8b93255ab39b4d3181
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557516
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71421}