Stop ResourceCallback from being an on-heap class

The first ResourceCallback::callbackHandler() call instantiates the
ResourceCallback singleton. As we call it in the pre-finalization
step, it should not be an on-heap class.

BUG=587663

Review-Url: https://codereview.chromium.org/2287483003
Cr-Commit-Position: refs/heads/master@{#414989}
1 file changed