Reuse WeakFactory from U2fDevice derived classes

Rather than having a separate WeakPtrFactory, reuse the
WeakPtrFactory from the derived classes when in the base U2fDevice
class.

BUG=686302

Review-Url: https://codereview.chromium.org/2842003002
Cr-Commit-Position: refs/heads/master@{#467079}
6 files changed