Declaring the weak_ptr_factory in proper order in src/cc.

Cleaning up weak_ptr_factory destruction order.
WeakPtrFactory should remain the last member so it'll be destroyed and
invalidate its weak pointers before any other members are destroyed.

BUG=303818

Review URL: https://codereview.chromium.org/581563002

Cr-Commit-Position: refs/heads/master@{#295279}
3 files changed