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According to the UMA metrics Mojo.Channel.WriteQueuePendingMessages at
least 22% of the time there are more than 2 messages queued while
flushing messages on POSIX systems.

This CL introduces a change where when there is more than one message
pending it will try to use writev(2). It's important to note that
we cannot use writev(2) when FileHandles are involved and it handles
that situation. However, according to Mojo.Channel.WriteMessageHandles
sending FileHandles is rare and 98.4% of the time messages do not
contain a FileHandle. So we should be able to benefit from writev(2)
frequently.

Bug: b:171251106
Change-Id: Ie632479da835c457d573ba4aa91bb22b8df084b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486501
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819939}