Add custom fuzzing logic for SharedMemoryHandle on Mac and Windows.
Add custom fuzzing logic for SharedMemoryHandle on Mac and Windows.

The fuzzer always creates an invalid SharedMemoryHandle. Previously, the
fuzzer couldn't create messages that contained a SharedMemoryHandle.

BUG=

Review URL: https://codereview.chromium.org/1729633002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#377104}

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