put webview with transparent background

fix flutter/flutter#29300

Author: jaumard

packages/webview_flutter/android/src/main/java/io/flutter/plugins/webviewflutter/FlutterWebView.java

@@ -6,6 +6,7 @@
 
 import android.annotation.TargetApi;
 import android.content.Context;
+import android.graphics.Color;
 import android.os.Build;
 import android.view.View;
 import android.webkit.WebStorage;
@@ -28,6 +29,7 @@ public class FlutterWebView implements PlatformView, MethodCallHandler {
   @SuppressWarnings("unchecked")
   FlutterWebView(Context context, BinaryMessenger messenger, int id, Map<String, Object> params) {
     webView = new WebView(context);
+    webView.setBackgroundColor(Color.TRANSPARENT);
     // Allow local storage.
     webView.getSettings().setDomStorageEnabled(true);
 
@@ -101,6 +103,7 @@ public void onMethodCall(MethodCall methodCall, Result result) {
   private void loadUrl(MethodCall methodCall, Result result) {
     String url = (String) methodCall.arguments;
     webView.loadUrl(url);
+    webView.setBackgroundColor(Color.TRANSPARENT);
     result.success(null);
   }
 

packages/webview_flutter/ios/Classes/FlutterWebView.m

@@ -66,6 +66,10 @@ - (instancetype)initWithFrame:(CGRect)frame
     configuration.userContentController = userContentController;
 
     _webView = [[WKWebView alloc] initWithFrame:frame configuration:configuration];
+    _webView.opaque = NO;
+    _webView.backgroundColor = UIColor.clearColor;
+    _webView.scrollView.backgroundColor = UIColor.clearColor;
+
     _navigationDelegate = [[FLTWKNavigationDelegate alloc] initWithChannel:_channel];
     _webView.navigationDelegate = _navigationDelegate;
     __weak __typeof__(self) weakSelf = self;
@@ -263,6 +267,9 @@ - (bool)loadUrl:(NSString*)url {
   }
   NSURLRequest* req = [NSURLRequest requestWithURL:nsUrl];
   [_webView loadRequest:req];
+  _webView.opaque = NO;
+  _webView.backgroundColor = UIColor.clearColor;
+  _webView.scrollView.backgroundColor = UIColor.clearColor;
   return true;
 }