Oilpan: Make CrossThreadPersistent not inherit from Persistent
Oilpan: Make CrossThreadPersistent not inherit from Persistent

This is an incremental step to land https://codereview.chromium.org/1121013003/.
This CL makes CrossThreadPersistent not inherit from Persistent.
The added code to CrossThreadPersistent is just a duplicate of the existing code in Persistent.

BUG=483380

Review URL: https://codereview.chromium.org/1143063002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195627 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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