Chrome sync: Put WeakPtrFactory members at the end

BUG=303818
R=akalin@chromium.org, nhiroki@chromium.org
TBR=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/26594002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227944 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227944