Only attmept to modify experiments key if it is present and valid.
Only attmept to modify experiments key if it is present and valid.

BUG=474417

Review URL: https://codereview.chromium.org/1213453002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#337211}

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