Enable Transparent IAM backgrounds

In order to support transparent IAM backgrounds we must set the WebView
to not opaque and set the background color to clear. We are always
doing this since the HTML will set its own background color unless it
is specified as clear by the IAM's creator.

Author: emawby

iOS_SDK/OneSignalSDK/Source/OSInAppMessageView.m

@@ -111,7 +111,8 @@ - (void)setupWebviewWithMessageHandler:(id<WKScriptMessageHandler>)handler {
 
     // Setup WebView, delegates, and disable scrolling inside of the WebView
     self.webView = [[WKWebView alloc] initWithFrame:mainBounds configuration:configuration];
-    
+    self.webView.backgroundColor = [UIColor clearColor];
+    self.webView.opaque = NO;
     [self addSubview:self.webView];
 
     [self layoutIfNeeded];