A callback object bound by BindToCurrentLoop runs on the target thread,
but the callback itself was not necessarily destroyed on the target
thread. That causes a data race when a non-thread-safe RefCounted objects
is bound to the callback. (E.g. a BindToCurrentLoop call for PPB_Buffer
in content::ContentDecryptorDelegate::DeliverFrame().)

This CL refactors BindToCurrentLoop and adds a clean up task to mitigate
the racy usage.

BUG=688072

Review-Url: https://codereview.chromium.org/2712603002
Cr-Commit-Position: refs/heads/master@{#454212}