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Previously, this class would fail gracefully if given a nullptr. Now it
has undefined behaviour. Dealing with null isn't necessary as no caller
passes nullptr here, and in general, pass-by-result style functions do
not expect null pointers. Dealing with nullptr makes my follow-up work
messy, so I am just removing it.

Also, moved constructor implementation to .cc file, and properly
documented the lifetime constraints on |json_string|.

BUG=455068

Review URL: https://codereview.chromium.org/889303004

Cr-Commit-Position: refs/heads/master@{#314703}