Use std::unique_ptr<> for WebTaskRunner::clone

Both Chromium and Blink code can use std::unique_ptr<> recently, so now
we can express the ownership passing on the type of WebTaskRunner::clone.
This CL replaces the return type of WebTaskRunner::clone from a raw pointer
to a std::unique_ptr<>, removes adoptClone, and updates the call-sites.

Review-Url: https://codereview.chromium.org/2148363004
Cr-Commit-Position: refs/heads/master@{#407178}
20 files changed