20542 Started stubbing in the FlutterView property of the mock FlutterViewControllers (cla: yes, platform-ios)

20568 Reland: Create PlatformView instance right after method channel call from Dart (cla: yes, platform-android, platform-ios)

20617 [embedder] Add FBO callback that takes frame info (cla: yes, platform-android, platform-ios, waiting for tree to go green)

20671 Enable ios platform view by default (cla: yes, platform-ios, waiting for tree to go green)

20672 [embedder] Add gl present callback that takes present info (cla: yes, platform-android, platform-ios, waiting for tree to go green)

20700 Avoid passing nil to IOS accessibility announcement  (cla: yes, platform-ios, waiting for tree to go green)