MessagePipe::Send() function uses a raw pointer, which does not claim the
ownership of the object. It's pretty error-prone, and should be avoid.
Considering we cannot send an "object" through the network, its bytes
representation is always needed. So this CL changes the parameter in
NamedMessagePipeHandler layer to use const-ref. A following CL will change the
MessagePipe::Send() function.

Bug: chromium:650926
Change-Id: Idf8104276718e900e61b35d095e80ce1c15e56bd
Reviewed-on: https://chromium-review.googlesource.com/644647


Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Zijie He <zijiehe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498903}