[Extensions Cleanup] Make Event::DeepCopy return a std::unique_ptr

Rather than returning a raw pointer with a comment saying "ownership is
passed", return a std::unique_ptr.

Bug: None

Change-Id: Id415847c7ca478f36ff3f624bd572b2c4678ac41
Reviewed-on: https://chromium-review.googlesource.com/c/1370646
Reviewed-by: Istiaque Ahmed <[email protected]>
Commit-Queue: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#615275}
4 files changed