Shortcut parsing of empty JSON
Shortcut parsing of empty JSON

Avoid calling heavyweight ParseJSON that could trigger
spawning a process if the received JSON is empty or
trivially empty dictionary.

BUG=490948

Review URL: https://codereview.chromium.org/1158783006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#331647}

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