Make FileWriterBase RefCountedGarbageCollectedWillBeGarbageCollectedFinalized

Now {,un}setPendingActivity is removed from FileWriter.
We can make this GarbageCollectedFinalized.

BUG=357163

Review URL: https://codereview.chromium.org/394623005

git-svn-id: svn://svn.chromium.org/blink/trunk@178130 bbb929c8-8fbe-4397-9dbb-9b2b20218538