Remove {,un}setPendingActivity from AudioContext.

This CL removes use of {,un}setPendingActivity from AudioContext. They were used to make the wrappers alive until Document is destroyed. This CL replaces the hack with a |hasPendingActivity()| impl.

Also, this removes ASSERT(m_isStopScheduled) from d-tor. This is never set when OfflineAudioContext::create() fails after creating an instance.

TEST=media/
BUG=354845

Review URL: https://codereview.chromium.org/208493002

git-svn-id: svn://svn.chromium.org/blink/trunk@170015 bbb929c8-8fbe-4397-9dbb-9b2b20218538