Track source handlers instead of nodes

|active_source_nodes_| tracked the active source nodes.  But we really
want to track the handlers for the nodes because that's what is really
running.  The nodes themselves may go away, but the handlers can still
be alive.

Thus, rename active_source_nodes_ to active_source_handlers_ and move it
to the DeferredTaskHandler class.

Bug: 752333
Change-Id: I1d7b39863f66782b6945fd5b7f02ab169c39584f
Reviewed-on: https://chromium-review.googlesource.com/c/1459791
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632660}
4 files changed