[WebMIDI] Add connection error handler for midi_client_

We check |midi_client_| before calling EndSession in the destructor of
MidiHost as it tells us if we've started a session before. |midi_client_|
could potentially be false if there was a connection error and could skip
the EndSession call.

Bug: 913270, 582328
Change-Id: Iececf880fcd61f1cc8ad65673e3dcfc9724ea5db
Reviewed-on: https://chromium-review.googlesource.com/c/1376089
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617079}
2 files changed