x11: Simplify some more event-rewrite code.

x11: Simplify some more event-rewrite code.

The WindowTreeHost implementation for X11 currently receives an event from X11
when the keyboard mapping changes in the system. The implementation then calls
into WindowTreeHost::OnKeyboardMappingChanged(), which triggers the
corresponding method in all WindowTreeHostObservers. The EventRewriter is a
WindowTreeHostObserver, and so it can then reset the keycodes it maintains.

But the EventRewriter is also a MessagePumpObserver, which means it already
receives the X11 MappingNotify event, and so the plumbing of the event through
aura is unnecessary.  There is no other consumer of
WindowTreeHostObserver::OnKeyboardMappingChanged(), so remove it from the api.

BUG=343639
R=oshima@chromium.org, sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255185

[sadrul, 2014-03-05 19:37:52]

oshima@ For chrome/browser/chromeos/ changes

sky@ For ui/aura/ ui/views/ changes

[sky, 2014-03-05 21:29:52]

nice, LGTM

[oshima, 2014-03-05 23:03:36]

lgtm

[sadrul, 2014-03-06 01:06:35]

Committed patchset #3 manually as r255185 (presubmit successful).