[fuchsia][scenic] Add input shield flag support in Flatland.

This CL adds support for "intercept_all_input" flag in
flutter_runner_config for flatland.

Test: flutter-embedder-test

Author: shivesh-ganju

shell/platform/fuchsia/flutter/flatland_external_view_embedder.cc

@@ -28,7 +28,9 @@ FlatlandExternalViewEmbedder::FlatlandExternalViewEmbedder(
     std::shared_ptr<FlatlandConnection> flatland,
     std::shared_ptr<SurfaceProducer> surface_producer,
     bool intercept_all_input)
-    : flatland_(flatland), surface_producer_(surface_producer) {
+    : flatland_(flatland),
+      surface_producer_(surface_producer),
+      intercept_all_input_(intercept_all_input) {
   flatland_->flatland()->CreateView2(
       std::move(view_creation_token), std::move(view_identity),
       std::move(view_protocols), std::move(parent_viewport_watcher_request));
@@ -336,6 +338,29 @@ void FlatlandExternalViewEmbedder::SubmitFrame(
       // Reset for the next pass:
       flatland_layer_index++;
     }
+
+    // TODO(fxbug.dev/104956): Setting hit regions for Flatland external view
+    // embedder should match with what is being done in GFX external view
+    // embedder.
+    //  Set up the input interceptor at the top of the scene, if applicable.  It
+    //  will capture all input, and any unwanted input will be reinjected into
+    //  embedded views.
+    if (intercept_all_input_) {
+      fuchsia::ui::composition::TransformId input_interceptor_node =
+          flatland_->NextTransformId();
+      flatland_->flatland()->CreateTransform(input_interceptor_node);
+      flatland_->flatland()->AddChild(root_transform_id_,
+                                      input_interceptor_node);
+
+      // Attach full-screen hit testing shield. Note that since the hit-region
+      // may be transformed (translated, rotated), we do not want to set
+      // width/height to FLT_MAX. This will cause a numeric overflow.
+      flatland_->flatland()->SetHitRegions(
+          input_interceptor_node,
+          {{{0, 0, kMaxHitRegionSize, kMaxHitRegionSize},
+            fuchsia::ui::composition::HitTestInteraction::
+                SEMANTICALLY_INVISIBLE}});
+    }
   }
 
   // Present the session to Scenic, along with surface acquire/release fences.

shell/platform/fuchsia/flutter/flatland_external_view_embedder.h

@@ -192,6 +192,9 @@ class FlatlandExternalViewEmbedder final
   SkISize frame_size_ = SkISize::Make(0, 0);
   float frame_dpr_ = 1.f;
 
+  // Set to true if input shield is enabled.
+  const bool intercept_all_input_;
+
   FML_DISALLOW_COPY_AND_ASSIGN(FlatlandExternalViewEmbedder);
 };