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In particular, the IsValid method had a confusing dual purpose - it was
used both to sanity check the inputs, and to check whether the instances
returned by the Get methods were null (i.e. Get failed). It has been
split into is_null() and DCheckValid(). Note that having DCheckValid as
a method also gives better stacktraces than the old DCHECK(IsValid())
style, since stacktraces will indicate which check failed.

BUG=458592

Review URL: https://codereview.chromium.org/1131303002

Cr-Commit-Position: refs/heads/master@{#329406}