Move ThreadableLoader to Oilpan heap (3/3)

This CL is a refactoring and does not change the behavior.

BUG=587663

Committed: https://crrev.com/2ec047e0061d6c07339e404f918f34459ab6a7b8
Review-Url: https://codereview.chromium.org/2193683004
Cr-Original-Commit-Position: refs/heads/master@{#409961}
Cr-Commit-Position: refs/heads/master@{#411919}
2 files changed