ThreadableLoader::cancel should be called before loader destruction

Currently ThreadableLoader's destructor implicitly cancels loading.
This CL requires a user to cancel the loader explicitly before
destructing a ThreadableLoader, as a preparation for moving it to
Oilpan heap.

BUG=587663

Review-Url: https://codereview.chromium.org/2146403004
Cr-Commit-Position: refs/heads/master@{#407074}
7 files changed