[WIP] ui/window: Master CL.

[WIP] ui/window: Master CL.

The plan is to introduce a PlatformWindow concept, which takes care of creating
and managing platform-dependent windows (e.g. Window on X11). It will have a
PlatformWindowDelegate to relay information it receives from the platform to.

The aura::WindowTreeHost becomes a concrete implementation, and the
platform-specific implementations will be removed. Instead, the WindowTreeHost
will own a PlatformWindow, and the WindowTreeHost becomes its
PlatformWindowDelegate.

In Ozone, a custom PlatformWindowCreator is installed that can create either an
X11 window, or any other window etc. The X11 backend for ozone (ozonex) creates
the X11 implementation of PlatformWindow, and so on.

This PlatformWindow will also be used in mojo for the native_viewport.

BUG=

[spang, 2014-04-28 16:41:31]

I think it is best if ui/window depends on ui/ozone, so that we can avoid the
funky and unexpected way ozone injects implementations into lower level
components.

From ui/window I think we should have something like

// platform_window_ozone.cc

PlatformWindow* CreatePlatformWindow(const gfx::Rect& bounds) { 
 return ui::OzonePlatform::CreateWindow(bounds);
}

This avoids needing a PlatformWindowCreator class, which is injected by the
platform, and which constitutes a hidden dependency from ui/window to ui/ozone.

To do this, all we need is to put the platform_window.h in a separate component.

  // Component: ui/window/types
  ui/window/types/platform_window.h

  // Component: ui/window
  ui/window/platform_window_x11.cc
  ui/window/platform_window_ozone.cc
  ui/window/platform_window_win.cc

  // Component: ui/ozone
  ui/ozone/ozone_platform.cc

Dependencies:
  ui/window -> ui/ozone
  ui/window -> ui/window/types
  ui/ozone -> ui/window/types

We've already used this pattern for ui/display.

[tfarina, 2014-04-28 18:20:03]

Sadrul, could you add a brief README file to ui/window?

[tfarina, 2014-04-28 18:20:52]

https://codereview.chromium.org/255823009/diff/1/ui/window/window.gyp
File ui/window/window.gyp (right):

https://codereview.chromium.org/255823009/diff/1/ui/window/window.gyp#newcode15
ui/window/window.gyp:15: '<(DEPTH)/base/base.gyp:base',
please, do not add more usage of <(DEPTH) in .gyp files. See my "recent" thread
on chromium-dev about this.

[tfarina, 2014-04-28 18:22:37]

https://codereview.chromium.org/255823009/diff/1/ui/window/platform_window.h
File ui/window/platform_window.h (right):

https://codereview.chromium.org/255823009/diff/1/ui/window/platform_window.h#...
ui/window/platform_window.h:26: class PlatformWindowDelegate {
do you mind moving this to platform_window_delegate.h?