Use WebRender to compute text index on click events

This is the second half of switching over to WebRender for hit testing.
Now that WebRender gives us the location of the hit tested point in the
display item, we can use that to calculate text index.

components/script_layout_interface/message.rs

@@ -120,7 +120,7 @@ pub enum ReflowGoal {
     ResolvedStyleQuery(TrustedNodeAddress, Option<PseudoElement>, PropertyId),
     OffsetParentQuery(TrustedNodeAddress),
     MarginStyleQuery(TrustedNodeAddress),
-    TextIndexQuery(TrustedNodeAddress, i32, i32),
+    TextIndexQuery(TrustedNodeAddress, Point2D<f32>),
     NodesFromPointQuery(Point2D<f32>, NodesFromPointQueryType),
 }