Create UnsafePtr to store on-heap pointers in Vector

In basic rule, we cannot store pointers for on-heap objects
in WTF::Vector.  But in some situation like weak processing,
we need it.
In such cases, you can use UnsafePtr to avoid validations.
Please check the pointee is guaranteed to be alive without
UnsafePtr<>.

BUG=515524

Review URL: https://codereview.chromium.org/1397073002

Cr-Commit-Position: refs/heads/master@{#354234}
5 files changed