ipc: Start host channel handshake earlier

BrowserChildProcessHostConnected notification, that's executed by
the runner loop, is too late as well to start the GPU and browser
process handshaking.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>

The patch was imported from https://github.com/tiagovignatti/ozone-wayland/
as of commit id 6922a29.

1)Revert changes related to EstablishChannel.
2)Ensure ChannelConnected flag is set properly. This would ensure
  that we don't add host as filter more than once.

Signed-off-by: Kalyan Kondapally <kalyan.kondapally@intel.com>
Update OzoneProcessObserver documentation.
patches: Track CL 32423009 from upstream

https://codereview.chromium.org/32423009
impl: Track CL 31043006 from upstream

https://codereview.chromium.org/31043006

It was a quite big rework in the DesktopRootWindowHost that needed many changes
in here. This commit also implements DesktopDragDropClientWayland, stubbing all
its functionality for now.