[WebMIDI] Check if accessor is in array before calling erase

The accessor may be in accessors_waiting_session_queue as SessionStarted
may not have been called when the document shuts down.

Bug: 915578, 915844, 582328
Change-Id: Ie7d864bcdaeb9dec9c3153ff1026de58d7aa8050
Reviewed-on: https://chromium-review.googlesource.com/c/1379920
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618234}
1 file changed