[webview_flutter] Update manifest file and remove unnecessary code

* Replace tabs to spaces in the manifest file
* Fix broken READEME.md
* Remove unnecessary code

Author: seungsoo47

packages/webview_flutter/README.md

@@ -16,6 +16,7 @@ To use this plugin, add below lines under the `<manifest>` section in your `tize
 <privileges>
   <privilege>http://tizen.org/privilege/internet</privilege>
 </privileges>
+```
 
 ## Usage
 

packages/webview_flutter/example/tizen/tizen-manifest.xml

@@ -6,8 +6,8 @@
         <icon>ic_launcher.png</icon>
         <metadata key="http://tizen.org/metadata/prefer_dotnet_aot" value="true"/>
     </ui-application>
-	<privileges>
-		<privilege>http://tizen.org/privilege/internet</privilege>
-	</privileges>
+    <privileges>
+        <privilege>http://tizen.org/privilege/internet</privilege>
+    </privileges>
     <feature name="http://tizen.org/feature/screen.size.all"/>
 </manifest>

packages/webview_flutter/lib/webview_flutter_tizen.dart

@@ -559,7 +559,6 @@ class _TizenPlatformTextureView extends LeafRenderObjectWidget {
     @required this.controller,
     @required this.hitTestBehavior,
     @required this.gestureRecognizers,
-    @required this.textureId,
   })  : assert(controller != null),
         assert(hitTestBehavior != null),
         assert(gestureRecognizers != null),
@@ -568,7 +567,6 @@ class _TizenPlatformTextureView extends LeafRenderObjectWidget {
   final TizenViewController controller;
   final PlatformViewHitTestBehavior hitTestBehavior;
   final Set<Factory<OneSequenceGestureRecognizer>> gestureRecognizers;
-  final int textureId;
 
   @override
   RenderObject createRenderObject(BuildContext context) => RenderTizenView(
@@ -591,8 +589,6 @@ class _TizenWebViewState extends State<TizenView> {
   TextDirection _layoutDirection;
   bool _initialized = false;
   FocusNode _focusNode;
-  int _textureId;
-  int get textureId => _textureId;
 
   static final Set<Factory<OneSequenceGestureRecognizer>> _emptyRecognizersSet =
       <Factory<OneSequenceGestureRecognizer>>{};
@@ -605,8 +601,8 @@ class _TizenWebViewState extends State<TizenView> {
       child: _TizenPlatformTextureView(
           controller: _controller,
           hitTestBehavior: widget.hitTestBehavior,
-          gestureRecognizers: widget.gestureRecognizers ?? _emptyRecognizersSet,
-          textureId: textureId),
+          gestureRecognizers:
+              widget.gestureRecognizers ?? _emptyRecognizersSet),
     );
   }