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Currently, blink creates its own isolates. We want to always use an
IsolateHolder to create the isolates. To be able to do this, I introduce
an Initialize method that setups V8. The new IsolateHolder ctor now
doesn't take any parameters but just creates new isolates according to
the configuration.

All non-blink gin users are cut over to the new API

BUG=none
R=abarth@chromium.org,andrewhayden@chromium.org,eroman@chromium.org

Review URL: https://codereview.chromium.org/553903003

Cr-Commit-Position: refs/heads/master@{#294262}