Merge the latest embedder header from flutter/engine (sony#222)

Merged from flutter/engine@0112d4b#diff-06f2c0a6c7bb5df337492f6d661883b12f847d1677220bd7dd7add48a78784c5

Author: Hidenori Matsubayashi

src/flutter/shell/platform/embedder/embedder.h

@@ -693,6 +693,14 @@ typedef enum {
 ///    released.
 ///  * All events throughout a key press sequence shall have the same `physical`
 ///    and `logical`. Having different `character`s is allowed.
+///
+/// A `FlutterKeyEvent` with `physical` 0 and `logical` 0 is an empty event.
+/// This is the only case either `physical` or `logical` can be 0. An empty
+/// event must be sent if a key message should be converted to no
+/// `FlutterKeyEvent`s, for example, when a key down message is received for a
+/// key that has already been pressed according to the record. This is to ensure
+/// some `FlutterKeyEvent` arrives at the framework before raw key message.
+/// See https://github.com/flutter/flutter/issues/87230.
 typedef struct {
   /// The size of this struct. Must be sizeof(FlutterKeyEvent).
   size_t struct_size;
@@ -706,11 +714,17 @@ typedef struct {
   ///
   /// For the full definition and list of pre-defined physical keys, see
   /// `PhysicalKeyboardKey` from the framework.
+  ///
+  /// The only case that `physical` might be 0 is when this is an empty event.
+  /// See `FlutterKeyEvent` for introduction.
   uint64_t physical;
   /// The key ID for the logical key of this event.
   ///
   /// For the full definition and a list of pre-defined logical keys, see
   /// `LogicalKeyboardKey` from the framework.
+  ///
+  /// The only case that `logical` might be 0 is when this is an empty event.
+  /// See `FlutterKeyEvent` for introduction.
   uint64_t logical;
   /// Null-terminated character input from the event. Can be null. Ignored for
   /// up events.