Fixes Issue #1140

Fixes https://github.com/react-native-community/react-native-webview/issues/1140
Based on a solution found at: https://stackoverflow.com/questions/56460362/how-to-force-wkwebview-to-ignore-hardware-silent-switch-on-ios
I changed the code found in the linked source from Swift to Objective-C, as required by this project. WARNING: I haven't used Swift before and very limited experience with Objective-C.

- For me this seems to work, but it is not the cleanest solution in my opinion.
- It might also be possible to play generated sound (i.e. using oscillator) instead of hardcoding the silent base64 mp3 data.
- Maybe ignoring silence switch should only be done if a parameter is supplied

ios/RNCWebView.m

@@ -133,6 +133,15 @@ static NSDictionary* customCertificatesForHost;
   }
 
 #if !TARGET_OS_OSX
+    [[NSNotificationCenter defaultCenter]addObserver:self
+    selector:@selector(appDidBecomeActive)
+        name:UIApplicationDidBecomeActiveNotification
+      object:nil];
+    
+    [[NSNotificationCenter defaultCenter]addObserver:self
+    selector:@selector(appWillResignActive)
+        name:UIApplicationWillResignActiveNotification
+      object:nil];
   if (@available(iOS 12.0, *)) {
     // Workaround for a keyboard dismissal bug present in iOS 12
     // https://openradar.appspot.com/radar?id=5018321736957952
@@ -156,6 +165,7 @@ static NSDictionary* customCertificatesForHost;
                                                selector:@selector(hideFullScreenVideoStatusBars)
                                                    name:UIWindowDidBecomeHiddenNotification
                                                  object:nil];
+      
   }
 #endif // !TARGET_OS_OSX
   return self;
@@ -1129,6 +1139,33 @@ static NSDictionary* customCertificatesForHost;
   }];
 }
 
+-(void)forceIgnoreSilentHardwareSwitch:(BOOL)initialSetup
+{
+    NSString *mp3Str = @"data:audio/mp3;base64,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";
+    NSString *scr;
+    if (initialSetup) {
+        scr = [NSString stringWithFormat:@"var s=new Audio('%@');s.id='wkwebviewAudio';s.controls=false;s.loop=true;s.play();document.body.appendChild(s);true", mp3Str];
+    } else {
+        scr = [NSString stringWithFormat:@"var s=document.getElementById('wkwebviewAudio');s.src=null;s.parentNode.removeChild(s);s=null;s=new Audio('%@');s.id='wkwebviewAudio';s.controls=false;s.loop=true;s.play();document.body.appendChild(s);true", mp3Str];
+    }
+    [self evaluateJS: scr thenCall: nil];
+}
+
+-(void)disableIgnoreSilentSwitch
+{
+    [self evaluateJS: @"document.getElementById('wkwebviewAudio').src=null;true" thenCall: nil];
+}
+
+-(void)appDidBecomeActive
+{
+    [self forceIgnoreSilentHardwareSwitch:false];
+}
+
+-(void)appWillResignActive
+{
+    [self disableIgnoreSilentSwitch];
+}
+
 /**
  * Called when the navigation is complete.
  * @see https://fburl.com/rtys6jlb
@@ -1136,11 +1173,13 @@ static NSDictionary* customCertificatesForHost;
 - (void)webView:(WKWebView *)webView
   didFinishNavigation:(WKNavigation *)navigation
 {
+    [self forceIgnoreSilentHardwareSwitch:true];
+    
    if (_injectedJavaScript) {
      [self evaluateJS: _injectedJavaScript thenCall: ^(NSString *jsEvaluationValue) {
        NSMutableDictionary *event = [self baseEvent];
        event[@"jsEvaluationValue"] = jsEvaluationValue;
-
+         
        if (self.onLoadingFinish) {
          self.onLoadingFinish(event);
        }