Implement autoplay DocumentActivation policy for Web Audio.

This is implementing the Web Audio side of the document activation
policy. The policy is not available by default and it might change
slightly but most likely, BaseAudioContext wouldn't have to change.

Bug: 715049
Change-Id: I61ba9331ab51c3765ec4731c096826e9a96b4758
Reviewed-on: https://chromium-review.googlesource.com/519322
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476642}
4 files changed