Audio thread should not access destination node

The AudioDestinationNode is an object managed by Oilpan so the audio
thread should not access it.  However, the audio thread needs
information (currentTime, etc) from the destination node. So instead
of accessing the audio destination handler (a scoped_refptr) via the
destination node, add a new member to the base audio context that
holds onto the destination handler.

The destination handler is not an oilpan object and lives at least as
long as the base audio context.

Bug: 860626, 860522, 863951
Test: Test case from 860522 doesn't crash on asan build
Change-Id: I3add844d4eb8fdc7e05b89292938b843a0abbb99
Reviewed-on: https://chromium-review.googlesource.com/1138974
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575509}