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The current BrokerClient is not signal-safe because of its use of base::Pickle
which uses new and delete, and also its use of std::vector, which is not
technically signal safe either. These are used in messaging IPCs with the
BrokerHost when a syscall is trapped, thus BrokerClient must be made signal
safe. This is discussed in https://crbug.com/255063.

This CL creates a new BrokerSimpleMessage class to handle simple IPC messaging
with strings and ints only. It uses fixed sized messages to simplify logic and
allow it to allocate all its memory on the stack. It creates send, receive, and
a synchronous send-and-receive method. It also adds a basic set of unit tests
to sanity check behavior.

Bug: 255063
Change-Id: I8077a515921b62969a1b8b173d903f2a118ed186
Reviewed-on: https://chromium-review.googlesource.com/553400
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558415}