Passes an IPC::Channel* to OnFilterAdded instead of IPC::Sender*.

This is changing so that filters can use more than just
the Sender interface of the Channel, and namely so they can get at
remote associated interfaces.

BUG=612500
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2349713003
Cr-Commit-Position: refs/heads/master@{#419329}