4566 [webview_flutter] Revert deprecation of clearCookies (cla: yes, p: webview_flutter, needs tests)

4567 [webview_flutter] Add a backgroundColor option to the webview platform interface (cla: yes, p: webview_flutter)

4569 [webview_flutter] Add a backgroundColor option to the Android webview (cla: yes, p: webview_flutter, platform-android)

4570 [webview_flutter] Add a backgroundColor option to the iOS webview (cla: yes, p: webview_flutter, platform-ios)

4573 [webview_flutter] Add loadRequest functionality to app facing package. (cla: yes, p: webview_flutter)

4576 [webview_flutter] Default Android to hybrid composition (cla: yes, p: webview_flutter)