[Ash] Introduces new IPC messages to support IME on Win-Ash

This CL introduces new IPC messages between the browser process and the metro_driver process
to support IME functionality on Windows Chrome running under Ash.

Why we need new IPC messages?:
In Ash mode on Windows, we need to use raw IME API (to be precise, Text Services Framework)
in the WinRT  process (hereafter we call it as "metro_driver" process) rather than the
browser process due to some API limitations. However most of platform-neutral, abstracted
IME event handling code are implemented in the browser process. To cope with this situation,
we will use a proxy mechanism between the metro_driver process and the browser process.
In fact, this is what we have been doing between the browser process and the renderer
processes.

Here is the entire WIP CL which describes how these new IPC will be used in the real world.
https://codereview.chromium.org/53553003/

BUG=164964
TEST=none

Review URL: https://codereview.chromium.org/74783010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237010 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237010