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Rather than nulling the WeakReferenceOwner's |flag_| field when it is
invalidated, recreate the Flag so that there is always a valid one.

This is a precursor to requiring manual re-creation of the Flag in order
to create new WeakPtrs after having called InvalidateWeakPtrs().

Bug: 927987
Change-Id: Idbaa89c288a1b61cd22d4a7039aa60a5ad916a24
Reviewed-on: https://chromium-review.googlesource.com/c/1460491
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632060}