ChannelMojo: Introduce thread-safe Send()

With the thread-safe Send(), ChannelProxy() calls Channel::Send()
from the UI thread instead of PostTask()-ing the Channel to the IO thread.
This eliminates one extra thread hop, improves the IPC latency.

Times taken to ping-pong 14kb messages 500k times using ChannelProxy:

 * ChannelPosix:       25,306ms +/- 2,012ms
 * ChannelMojo before: 27,870ms +/- 4,205ms
 * ChannelMojo now:    19,758ms +/- 2,074ms

BUG=272113,473367
R=viettrungluu@chromium.org, agl@chromium.org

Review URL: https://codereview.chromium.org/1054943005

Cr-Commit-Position: refs/heads/master@{#325911}

• ipc/ipc_channel.h[diff]
• ipc/ipc_channel_common.cc[diff]
• ipc/ipc_channel_proxy.cc[diff]
• ipc/ipc_channel_proxy.h[diff]
• ipc/mojo/ipc_channel_mojo.cc[diff]
• ipc/mojo/ipc_channel_mojo.h[diff]
• ipc/mojo/ipc_message_pipe_reader.cc[diff]
• ipc/mojo/ipc_message_pipe_reader.h[diff]