IPC fuzzer: Update a call to IPC::Message::FindNext.
IPC fuzzer: Update a call to IPC::Message::FindNext.

R=erikchen@chromium.org

Review URL: https://codereview.chromium.org/1312793005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345198}

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