ChannelProxy: Send() from UI thread when ChannelMojo is used.

ChannelProxy: Send() from UI thread when ChannelMojo is used.

This change introduce the notion of thread-safe send to
ChannelProxy. It calls Channel::Send() from UI thread instead
of posting a task to the IO thread, elimating the overhead.

This is possible as Mojo API is thread safe. Some local test
shows that this (and coming Mojo-side change) offsets the
overhead of extra copying incurred by Mojo-IPC integration
in some benchmark.

R=viettrungluu@chromium.org, agl@chromium.org
BUG=377980

[Hajime Morrita, 2014-12-05 02:54:09]

Trung: PTAL for ipc/mojo
Adam: For ipc/

https://codereview.chromium.org/780223002/diff/1/ipc/mojo/ipc_channel_mojo.cc
File ipc/mojo/ipc_channel_mojo.cc (left):

https://codereview.chromium.org/780223002/diff/1/ipc/mojo/ipc_channel_mojo.cc...
ipc/mojo/ipc_channel_mojo.cc:178: return false;
I do this just for exercising tests. No intention to do this in this CL.

[Hajime Morrita, 2014-12-05 22:30:11]

Hmm. Apparently my measurement was wrong. The impact of this change isn't clear
as I thought. I'll do more test.
Closing for now. I'm sorry for the noise.