Fix cross-world Promise leak in navigator.keyboard.getLayoutMap()

This CL refactors KeyboardLayout::GetKeyboardLayoutMap to use
ScriptPromiseProperty instead of maintaining a HeapVector of
ScriptPromiseResolvers.

This change aligns the implementation with Blink's preferred patterns
for Promise-returning methods while continuing to satisfy the
Keyboard API specifications:
  - Concurrent requests within the same world are automatically handled
    by ScriptPromiseProperty to return the same pending promise
    instance.
  - To ensure sequential calls return new promise instances (as
    required by the spec), `layout_map_property_` is now explicitly
    cleared once it has reached a resolved or rejected state.

Bug: 501485453
Change-Id: Ia19bc3408e61ec160764f6229b6158fa5a912c3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7751051
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1615641}

Author: joedow-42

keyboard-map/navigator-keyboard-map-two-sequential-requests.https.html

@@ -5,10 +5,12 @@
 'use strict';
 
 promise_test(() => {
-  return navigator.keyboard.getLayoutMap()
-      .then(() => {
-        return navigator.keyboard.getLayoutMap();
-      });
-}, '[Keyboard Map] getLayoutMap() called twice sequentially');
+  const p1 = navigator.keyboard.getLayoutMap();
+  return p1.then(() => {
+    const p2 = navigator.keyboard.getLayoutMap();
+    assert_not_equals(p1, p2);
+    return p2;
+  });
+}, '[Keyboard Map] getLayoutMap() returns a new promise for sequential calls');
 
 </script>