This ensures that platform is initialized before the isolate and
properly restores the previous platform at the end of the test.

Bug: 
Change-Id: I2771b7538362c400c5ff61411222beb7d7e62b02
Reviewed-on: https://chromium-review.googlesource.com/707111
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48405}