Make FileReader RefCountedWillBeGarbageCollectedFinalized

Before this patch, FileReader was RefCountedWillBeRefCountedGarbageCollected.
We have removed {,un}setPendingActivity from FileReader, so we can now make
this GarbageCollectedFinalized.

BUG=357163

Review URL: https://codereview.chromium.org/393703003

git-svn-id: svn://svn.chromium.org/blink/trunk@178133 bbb929c8-8fbe-4397-9dbb-9b2b20218538