Remove ActiveDOMObject::willStop, and introduce WorkerGlobalScope::TerminationObserver.

ActiveDOMObject::willStop is called in the main thread, and it's very confusing
because other functions are called in ActiveDOMObject's context thread.

Only synchronous Web SQL Database needs willStop.  This CL inroduces a dedicated
interface to notify termination request to a Worker, and DatabaseContext
implements it.

BUG=352621

Review URL: https://codereview.chromium.org/248523003

git-svn-id: svn://svn.chromium.org/blink/trunk@172357 bbb929c8-8fbe-4397-9dbb-9b2b20218538