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- Prepare V8PerIsolateData for holding V8HeapController.
- Harden existing tests to swap in the expected visitor/controller. This way it
  is possible to test ScriptWrappableVisitor while already running on a different
  embedder heap tracer.

Change-Id: I169de233a4dd9886d153e7d97ca63e7caee0d809
Bug: chromium:843903
Reviewed-on: https://chromium-review.googlesource.com/1160656
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580892}