Web MIDI: Check valid clients in MidiManager

MidiService had a |active_clients_| to detect when it destructs
MidiManager instance, but since MidiManager has a complete set
of active clients, it would be simple and safe to ask MidiManager
to check if there is at least one active client.

Bug: 746969, 750011, 747662
Change-Id: I1491df4bc4b20c2605c979babb11e813d25702e4
Reviewed-on: https://chromium-review.googlesource.com/622294
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495918}
4 files changed