Make ActiveDOMObject::setPendingActivity() and unsetPendingActivity() protected.

With this, we can safely override hasPendingActivity() without worrying about
other code calling setPendingActivity() and unsetPendingActivity() expecting
control of hasPendingActivity()'s return value.

BUG=none

Review URL: https://codereview.chromium.org/70193002

git-svn-id: svn://svn.chromium.org/blink/trunk@161885 bbb929c8-8fbe-4397-9dbb-9b2b20218538