[iOS] Mark EmbeddedViewCount const (#54062)

Since this method isn't mutating the pool, we should mark it const.

Cleanup spotted while working on https://github.com/flutter/engine/pull/54056.

No test because no semantic change. The compiler is the test.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

diff --git a/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm b/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm
index 551be71..c3c76ec 100644
--- a/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm
+++ b/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm
@@ -398,7 +398,7 @@
   views_to_recomposite_.insert(view_id);
 }
 
-size_t FlutterPlatformViewsController::EmbeddedViewCount() {
+size_t FlutterPlatformViewsController::EmbeddedViewCount() const {
   return composition_order_.size();
 }
 

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h

diff --git a/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h b/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h
index 6373e1f..afda710 100644
--- a/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h
+++ b/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h
@@ -234,7 +234,7 @@
   void PrerollCompositeEmbeddedView(int64_t view_id,
                                     std::unique_ptr<flutter::EmbeddedViewParams> params);
 
-  size_t EmbeddedViewCount();
+  size_t EmbeddedViewCount() const;
 
   // Returns the `FlutterPlatformView`'s `view` object associated with the view_id.
   //