[WebAudio DevTools] Report graph connection events

This is a follow-up of:
https://chromium-review.googlesource.com/c/chromium/src/+/1724499

This CL introduces new reporting methods to the AudioGraphTracer
class to notify when the graph object connection/disconnection to
the devtool frontend.

The actual reporting code to InspectorWebAudioAgent will be implemented
by the follow-up CLs.

Bug: 970898
Change-Id: Idd00e73f8aec36f2812c4ee60b6bf42d1a2bbd39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726957
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683210}