Implement Linux IME support

mojo/services/native_viewport/interfaces/native_viewport.mojom

Index: mojo/services/native_viewport/interfaces/native_viewport.mojom
diff --git a/mojo/services/native_viewport/interfaces/native_viewport.mojom b/mojo/services/native_viewport/interfaces/native_viewport.mojom
index 34654146fc1560d0ad1c496f9eb319e1c38b3e1c..99e2bad3f41cea5407d78a139ed9efc5ea9c3f77 100644
--- a/mojo/services/native_viewport/interfaces/native_viewport.mojom
+++ b/mojo/services/native_viewport/interfaces/native_viewport.mojom
@@ -34,6 +34,7 @@ interface NativeViewport {
   Close();
   SetSize(Size size);
   SetEventDispatcher(NativeViewportEventDispatcher dispatcher);
+  SetKeyEventDispatcher(NativeViewportEventDispatcher dispatcher);

[jamesr, 2015/11/19 23:20:18]

The naming here is very strange - what's the difference between "SetEvent" and
"SetKeyEvent"? Are key events not events?  It's possible that the first method
should be called SetPointerEventDispatcher() but that raises another issue. 
Splitting different types of events at the NativeViewport level means that it's
impossible for things further on in the system to reconstruct the original order
(unless the event protocol provides additional information).  When key events
are dispatched to an IME abstraction the original order probably doesn't matter
that much but when they aren't the order definitely does matter and
NativeViewport is the wrong layer to make a decision about which matters.  I
think that splitting the stream must be done at a higher level of the stack that
has more information about the intended use case.  For now, that means in
flutter (or other apps that talk directly to native viewport).  When mozart is
further along the view manager system will be able to make this decision.

Concretely for this patch I think you should send key events down the current
(one) NativeViewportEventDispatcher interface and then forward the key events to
the IME from flutter.  This means having another pipe and copy of the events for
now, but once the view manager is performing the dispatching the extra cost will
go away (since the view manager will have to multiplex anyway)

 
   // Requests a ContextProvider capable of producing contexts that draw to
   // this native viewport.