28495 Fixes accessibility issue that bridge fails to clean up FlutterScrollableSemanticsObject (platform-ios, cla: yes, waiting for tree to go green)

28500 backdrop_filter_layer only pushes to the leaf_nodes_canvas (platform-ios, cla: yes, waiting for tree to go green)

28520 [flutter_releases] apply patches to flutter-2.5-candidate.8 (platform-ios, platform-android, cla: yes, platform-fuchsia, embedder)

28535 [flutter_releases] flutter-2.6-candidate.3 Engine Cherrypicks (platform-ios, platform-android, cla: yes, platform-fuchsia, embedder)

28658 [shared_engine] Avoid the method surfaceUpdated is called many times when multiple FlutterViewController shared one engine (platform-ios, cla: yes, waiting for tree to go green)