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This CL adds |currentFrame| property to AudioWorkletGlobalScope according
to: https://github.com/WebAudio/web-audio-api/pull/1493.

1. Previously the time stamp (currentTime) was updated when the first
   worklet processor gets called, and it was inconsistent with how it is
   updated in the other nodes. Now updating the frame number is
   streamlined, so the worklet processor also gets the same treatment.

2. In order to streamline this process, WorkerThread reference was
   added to BaseAudioContext. Due to the GC rule, BaseAudioContext cannot
   keep a reference of WorkletGlobalScope, so it always derives the
   global scope from the worker thread.

Bug: 814794
Test: http/tests/webaudio/audio-worklet/timing-info.html
Change-Id: I4b313377d80f8678c473cf788211e373fd1644cb
Reviewed-on: https://chromium-review.googlesource.com/935157
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539300}