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This forces WeakPtrs to be invalidated before other members' destructors run.
See the bug or the documentation of WeakPtrFactory for more information. I'm
in the process of trying to turn on a clang check for this (see the bug).

BUG=303818
TBR=qinmin@chromium.org

Review URL: https://codereview.chromium.org/841453006

Cr-Commit-Position: refs/heads/master@{#311506}