4481 [webview_flutter] Update webview_flutter documentation (cla: yes, waiting for tree to go green, p: webview_flutter)

4486 [webview_flutter] Implementations of loadFile and loadHtmlString for WKWebView (cla: yes, waiting for tree to go green, p: webview_flutter, platform-ios)

4489 [webview_flutter] Update Widget Unit Tests to be platform agnostic (cla: yes, p: webview_flutter)