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When setting the panning model, the Panner object gets replaced with
a new Panner for the new panning model.  This happens with the process
lock.  However, in AudioNode::ProcessIfNecessary, there is a call to
PropagatesSilence() that calls TailTime() and LatencyTime() in the
audio thread and this is not covered by the process lock. Hence
TailTime() may get called on a destroyed Panner object.

So let PannerNode implement its own ProcessIfNecessary method to
add a process lock to protect PropagatesSilence (and thus
TailTime/LatencyTime). This requires making ProcessIfNecessary virtual,
and requires making a few member variables protected instead of
private.


Bug: 1023817
Change-Id: Icc850d9f8b952814c40c29912cdec3d25d6f4ae2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1924718Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717029}