android_webview  : Class member variable WeakPtrFactory in proper order.

Maintaing the proper WeakPtrFactory destruction order to avoid the runtime
issue like unrleased memory,etc...
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/509453002

Cr-Commit-Position: refs/heads/master@{#294569}
5 files changed