Make GetRootWindow() return a Window instead of a RootWindow.

Make GetRootWindow() return a Window instead of a RootWindow.

Also adds a typedef from RootWindow to WindowEventDispatcher, the eventual name of this type.
Adds a GetDispatcher() method that crawls up to the root Window and returns the WindowEventDispatcher.

R=sky@chromium.org
http://crbug.com/308843

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=231077

[Ben Goodger (Google), 2013-10-24 20:49:34]

There are probably many more places where we could use Window instead of
RootWindow/WED, but this change is already humongous so I am going to try to
avoid solving any of those in this iteration.

[sky, 2013-10-24 20:59:45]

https://codereview.chromium.org/37733003/diff/80001/ui/aura/root_window.cc
File ui/aura/root_window.cc (right):

https://codereview.chromium.org/37733003/diff/80001/ui/aura/root_window.cc#ne...
ui/aura/root_window.cc:152: set_dispatcher(this);
You need to set this to NULL in the destructor, otherwise ~Window is going to
use it.

[sky, 2013-10-24 21:02:08]

LGTM when set_dispatcher(NULL) added

[Ben Goodger (Google), 2013-10-25 19:22:53]

Committed patchset #13 manually as r231077 (presubmit successful).