Remove ActiveDOMObject::{,un}setPendingActivity

Now all invocation of {,un}setPendingActivity is gone!
Removing them from ActiveDOMObject.

The default implementation of hasPendingActivity is still left, as there
are some classes deriving from ActiveDOMObject to receive suspend() and
resume() notifications.

BUG=354845

Review URL: https://codereview.chromium.org/401603003

git-svn-id: svn://svn.chromium.org/blink/trunk@178368 bbb929c8-8fbe-4397-9dbb-9b2b20218538