Add elevation and thickness to SemanticsNode (#7282)

lib/ui/semantics.dart

@@ -607,6 +607,14 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
   ///
   /// The `transform` is a matrix that maps this node's coordinate system into
   /// its parent's coordinate system.
+  ///
+  /// The `elevation` describes the distance in z-direction between this node
+  /// and the `elevation` of the parent.
+  ///
+  /// The `thickness` describes how much space this node occupies in the
+  /// z-direction starting at `elevation`. Basically, in the z-direction the
+  /// node starts at `elevation` above the parent and ends at `elevation` +
+  /// `thickness` above the parent.
   void updateNode({
     int id,
     int flags,
@@ -618,6 +626,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
     double scrollPosition,
     double scrollExtentMax,
     double scrollExtentMin,
+    double elevation,
+    double thickness,
     Rect rect,
     String label,
     String hint,
@@ -647,6 +657,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
       rect.top,
       rect.right,
       rect.bottom,
+      elevation,
+      thickness,
       label,
       hint,
       value,
@@ -674,6 +686,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
     double top,
     double right,
     double bottom,
+    double elevation,
+    double thickness,
     String label,
     String hint,
     String value,

lib/ui/semantics/semantics_node.h

@@ -87,6 +87,8 @@ struct SemanticsNode {
   double scrollPosition = std::nan("");
   double scrollExtentMax = std::nan("");
   double scrollExtentMin = std::nan("");
+  double elevation = 0.0;
+  double thickness = 0.0;
   std::string label;
   std::string hint;
   std::string value;

lib/ui/semantics/semantics_update_builder.cc

@@ -50,6 +50,8 @@ void SemanticsUpdateBuilder::updateNode(
     double top,
     double right,
     double bottom,
+    double elevation,
+    double thickness,
     std::string label,
     std::string hint,
     std::string value,
@@ -72,6 +74,8 @@ void SemanticsUpdateBuilder::updateNode(
   node.scrollExtentMax = scrollExtentMax;
   node.scrollExtentMin = scrollExtentMin;
   node.rect = SkRect::MakeLTRB(left, top, right, bottom);
+  node.elevation = elevation;
+  node.thickness = thickness;
   node.label = label;
   node.hint = hint;
   node.value = value;

lib/ui/semantics/semantics_update_builder.h

@@ -38,6 +38,8 @@ class SemanticsUpdateBuilder
                   double top,
                   double right,
                   double bottom,
+                  double elevation,
+                  double thickness,
                   std::string label,
                   std::string hint,
                   std::string value,