Oilpan: WebAudio: Fix unsafe access to HashSet objects in AudioContext::trace.

- We don't need to trace m_dirtySummingJunctions, which is an off-heap HashSet.
- It's very difficult to trace on-heap m_dirtyAudioNodeOutputs correctly
  because we always need graph lock. This CL reverts it to off-heap
  HashSet.
- Added some comments, and an assertion.

BUG=394294
R=ager@chromium.org, haraken@chromium.org, wibling@chromium.org

Review URL: https://codereview.chromium.org/399823002

git-svn-id: svn://svn.chromium.org/blink/trunk@178340 bbb929c8-8fbe-4397-9dbb-9b2b20218538