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Adds fuzzers for Channel and NodeChannel, the two main primitives
over which Mojo transmits all IPC messages. Both objects define and
parse different layers of the internal control protocol.

The Channel fuzzer just fires random data at a platform-specific
Channel implementation, covering the lowest protocol layer which
parses a Channel::Message header.

The NodeChannel fuzzer sends well-formed Channel::Message header
data with a fuzzed payload to a NodeChannel receiver, which covers
parsing the rest of the internal control protocol.

Bug: 900440
Change-Id: I9fe9b88fc3bced17427db28daad931fb64a44280
Reviewed-on: https://chromium-review.googlesource.com/c/1352828
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611583}