Defining a clear life cycle of BaseAudioContext and its derivatives requires a in-depth discussion, so first I would like to focus on the shutting down process.

1. User closes the context.
  - The explicit closure of context means that the resource can be collected.
  - However, the destination node must be valid.

2. User drops the reference to the context.
  - The context will leak unless it is closed.
  - When GC happens, BaseAudioContext::HasPendingActivity() gets called.

3. User closes the tab.
  - AudioContext::ContextDestroyed() gets called.
  - Eventually it will be chained to BaseAudioContext::ContextDestroyed().

Currently there is nothing we can do about 2., but 1. and 3. can be resolved.

Solution 1.
 - Stop the destination node rendering.
 - Do not delete the destination node. It will eventually be collected when
   ContextDestroyed() is called.

Solution 3.
 - Implement AudioContext::ContextDestroyed().

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fbeb7bf25a794a2d482e56faab4577b09a36c9d5

commit fbeb7bf25a794a2d482e56faab4577b09a36c9d5
Author: Hongchan Choi <hongchan@chromium.org>
Date: Fri Jun 29 01:08:08 2018

Add AudioContext::ContextDestroyed

Currently AudioContext has no way of uninitializing the instance without
an explicit context closure. This extends BaseAudioContext's
ContextDestroyed to AudioContext so we can hook up its own uninitialize
routine, and then eventually calls the parent's uninitializer.

This also fixes other issue of UKM metrics being reported in less cases
because they were only reported when context.close() was called.

Bug:  856690 ,  688052 ,  670065 
Change-Id: I721dc2c27201caab1b0a95baf8f6906f44b76979
Reviewed-on: https://chromium-review.googlesource.com/1115516
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571349}
[modify] https://crrev.com/fbeb7bf25a794a2d482e56faab4577b09a36c9d5/third_party/WebKit/LayoutTests/webaudio/internals/audiocontext-close.html
[modify] https://crrev.com/fbeb7bf25a794a2d482e56faab4577b09a36c9d5/third_party/blink/renderer/modules/webaudio/audio_context.cc
[modify] https://crrev.com/fbeb7bf25a794a2d482e56faab4577b09a36c9d5/third_party/blink/renderer/modules/webaudio/audio_context.h
[modify] https://crrev.com/fbeb7bf25a794a2d482e56faab4577b09a36c9d5/third_party/blink/renderer/modules/webaudio/base_audio_context.cc
[modify] https://crrev.com/fbeb7bf25a794a2d482e56faab4577b09a36c9d5/third_party/blink/renderer/modules/webaudio/base_audio_context.h