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If a WeakPtr references an already-destroyed object, operator-> and
operator* end up simply dereferencing nullptr. However, dereferencing
nullptr is undefined behavior and can be optimized in surprising ways
by compilers. To prevent this from happening, add a defence of last
resort and CHECK that the WeakPtr is still valid.

Bug: 817982
Change-Id: Ib3a025c18fbd9d5db88770fced2063135086847b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463857
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816701}