Events: Introduce interface for event conversion.

Dispatcher is responsible for event conversion and also
for dispatching native events. We should move event conversion
out of dispatcher.This CL introduces the following classes:
1) EventConverterOzoneWayland: This class is same as
   WaylandDispatcherDelegate, with same responsibility.
2) EventConverterInProcess: This class is same as
   BrowserProcessDispatcherDelegate, with same responsibility
   and interface.
3) RemoteEventDispatcher: This class is same as
   GpuProcessDispatcherDelegate, with same responsibility and
   interface.

These classes will be taken into use in followup patches.
Use EventConverterOzoneWayland to convert native events to ui::Aura.

This CL takes into use EventConverterOzoneWayland. The changes are
pretty straightforward.
Dispatcher: Remove un-necessary apis from Dispatcher.

This patch removes all apis related to events and
WaylandDispatcherDelegate. EventConverterOzoneWayland is now
responsible for providing this functionality.
Dispatcher: Remove unnecessary classes.

This CL removes DispatcherDelegate and it's derived classes.