Adds RefPtr::leakRef method to allow raw pointers that prevent destruct.

Part of the removal of PassRefPtr.

Adds RefPtr::leakRef method which returns a raw pointer while
maintaining a ref to the RefCounted object. This creates a hanging ref
on the object.

RefPtr::leakRef is intended to replace PassRefPtr::leakRef.

Also replaces all conversions of RefPtr objects to PassRefPtr objects
in order to use PassRefPtr::leakRef (i.e. through .release().leakRef())
with direct usage of the new RefPtr::leakRef method.

BUG=494719

Review-Url: https://codereview.chromium.org/2696703008
Cr-Commit-Position: refs/heads/master@{#451658}
5 files changed