Allow Window implementations to indicate that they don't support a clipboard.

This is important for the SERVO_HEADLESS configuration, because
creating a clipboard on linux creates an X context which then causes
reftest instability.

ports/cef/window.rs

@@ -451,6 +451,10 @@ impl WindowMethods for Window {
             }
         }
     }
+
+    fn supports_clipboard(&self) -> bool {
+        true
+    }
 }
 
 struct CefCompositorProxy {

ports/glutin/window.rs

@@ -614,6 +614,10 @@ impl WindowMethods for Window {
             _ => {}
         }
     }
+
+    fn supports_clipboard(&self) -> bool {
+        true
+    }
 }
 
 /// The type of a window.
@@ -718,6 +722,10 @@ impl WindowMethods for Window {
     /// Helper function to handle keyboard events.
     fn handle_key(&self, _: Key, _: constellation_msg::KeyModifiers) {
     }
+
+    fn supports_clipboard(&self) -> bool {
+        false
+    }
 }
 
 struct GlutinCompositorProxy {

ports/gonk/src/window.rs

@@ -841,6 +841,10 @@ impl WindowMethods for Window {
     fn prepare_for_composite(&self, _width: usize, _height: usize) -> bool {
         true
     }
+
+    fn supports_clipboard(&self) -> bool {
+        true
+    }
 }
 
 struct GonkCompositorProxy {