win: Move WeakPtrFactory towards the end of a class.

The class has another WeakPtrFactory member, but the two factories are used
in different ways (one has explicit calls to InvalidateWeakPtrs()) so they
can't be merged.  I had missed that there's a second WeakPtrFactory in
https://codereview.chromium.org/1107353003/

Should fix the last

  [chromium-style] WeakPtrFactory members which refer to their outer class must
  be the last member in the outer class definition.

warning on Windows.

BUG=303818,467287

Review URL: https://codereview.chromium.org/1121733002

Cr-Commit-Position: refs/heads/master@{#327948}
2 files changed