Replace uses of {un}setPendingActivity() in FetchBodyStream.

Follow up r178002 and provide FetchBodyStream::hasPendingActivity() rather
than updating ActiveDOMObject's refcount-based pending counts.

This lets the class derive from the simpler Oilpan GC class
(GarbageCollectedFinalized) rather than RefCountedGarbageCollected.

R=haraken@chromium.org,kouhei@chromium.org
BUG=354845

Review URL: https://codereview.chromium.org/390673002

git-svn-id: svn://svn.chromium.org/blink/trunk@178009 bbb929c8-8fbe-4397-9dbb-9b2b20218538